American  ^ature 

Group  III.    The  Functions  of  Nature 


THE  LIVING  PLANT 


A  DESCRIPTION  AND  INTERPRETATION  OF  ITS 
FUNCTIONS  AND  STRUCTURE 


BY 
WILLIAM  F.  GANONG,  PH.D. 

PROFESSOR  OF  BOTANY  IN  SMITH  COLLEGE 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 
1913 


COPYRIGHT,  1913, 

BY 
HENRY  HOLT  AND  COMPANY 


Published  April,  1913. 


HESS  OK  T.   MORKY   &   SON, 


Of  the  scholars  of  Salomon's  House, — "lastly,  we  have 
three  that  raise  the  former  discoveries  by  experiments, 
into  greater  observations,  axioms,  and  aphorisms.  These 
we  call  Interpreters  of  Nature." 

FRANCIS  BACON,  The  New  Atlantis 


2054835 


PREFACE 

The  very  first  words  I  would  write  in  this  book  are  addressed 
to  my  botanical  colleagues,  whom  I  wish  to  inform  that  the  work 
is  not  intended  for  them.  In  this  statement  I  am  by  no  means 
invoking  immunity  from  scientific  criticism,  but  emphasizing  the 
aim  of  the  book.  It  is  not  designed  as  a  digest  of  our  present 
scientific  knowledge  of  plant  physiology  for  the  use  of  experts  in 
that  subject,  but,  in  conformity  with  the  aim  of  the  series  of  which 
it  is  a  part,  it  seeks  to  present  to  all  who  have  interest  to  learn  an 
accurate  and  vivid  conception  of  the  principal  things  in  plant  life. 
I  was  once  myself  such  a  learner,  and  I  have  tried  to  write  such  a 
book  as  I  would  then  have  delighted  to  read.  It  is,  in  a  word,  an 
attempt  at  that  literature  of  interpretation  which  was  fore- 
shadowed by  Francis  Bacon  in  the  fine  passage  that  stands  on  its 
dedicatory  page. 

This  aim  will  explain  peculiarities  of  the  work  not  otherwise 
obvious.  Thus,  I  have  been  at  more  pains  to  be  clear  than  to  be 
brief,  assuming  on  the  part  of  my  reader  no  great  knowledge  of 
the  subject,  but  a  large  willingness  to  take  trouble  to  learn;  and  as 
I  have  tried  to  discuss  every  process  with  fulness  enough  to  eluci- 
date its  nature,  my  book  has  wandered  through  a  leisurely  course 
to  a  length  quite  shockingly  great.  But  I  comfort  myself  with  the 
reflection  that  the  plan  and  the  subject  hardly  permit  other  treat- 
ment; for  a  royal  road  to  a  real  understanding  of  plant  phenomena 
does  neither  exist  nor  can  it  be  built.  Perhaps,  indeed,  the  very 
portliness  of  the  volume  will  act  as  a  deterrent  to  any  attempt  at 
a  desultory  reading  in  the  hammock,  and  will  rather  suggest  the 
study  table,  and  the  principal  feature  of  an  evening's  business, 


vi  Preface 

and  sternly-preserved  leisure  for  reflective  concentration  on  the 
matters  it  considers.  At  least,  any  value  it  may  have  for  the 
reader  will  be  realized  best  through  this  mode  of  approach. 

As  to  the  method  of  treatment  in  particular,  I  have  sought 
especially  to  interpret  those  phenomena  of  plant  life  which  come 
within  ordinary  observation  and  experience,  penetrating  just 
deeply  enough  into  each  to  make  clear  the  principle  of  its  opera- 
tion,— "the  theory  of  the  thing"  in  popular  phrase; — and  some- 
times that  has  taken  me  far  and  sometimes  it  has  not.  Thus  is 
explained  the  absence  of  some  matters  of  high  technical  interest, 
which  lie,  however,  outside  the  experience  of  the  general  observer. 
Where  explanations  are  concerned,  I  have  given  the  known  ones 
when  there  are  any,  and  when  these  are  lacking  I  have  not 
hesitated  to  supply  suggestions  of  my  own,  though  in  a  way 
designed  to  show  their  hypothetical  character.  As  to  statements 
of  fact,  I  have  meant  to  present  only  those  which  have  acquired 
the  impersonal  validity  of  science,  for  which  reason  I  have  omitted 
a  good  many  of  the  newest  ideas,  even  at  the  risk  of  seeming  not 
to  know  them;  for  I  have  noticed  that  he  who  is  too  closely  up  to 
date  in  science  has  later  a  good  deal  to  unlearn. 

This  deliberate  conservatism  is  not,  however,  the  inspiration 
of  my  advocacy  of  Darwinian  adaptation,  for  that  is  based  upon 
conviction  as  to  its  essential  correctness.  I  am  very  well  aware 
that  some  eminently  respectable  people  now  consider  adaptation, 
except  as  an  accident,  an  antiquated  idea.  I  have  myself  expe- 
rienced periods  of  this  belief,  but  have  always  found  myself  back 
to  causative  adaptation  as  the  most  rational  explanation  we 
possess  of  the  relations  of  living  beings  to  their  environment. 
But  while  holding  to  the  reality  of  adaptation  as  an  historical  and 
causative  process,  I  do  not  by  any  means  suppose  that  all  plant 
phenomena  are  explainable  on  this  basis;  and  in  this  book  I  have 
tried  to  sort  out  the  numerous  influences  at  work,  and  to  show 
which  phenomena  are  best  explained  by  adaptation,  which  by 
mechanical  causation,  and  which  by  others  of  the  possible  forma- 


Preface  vii 

tive  influences.  But  adaptation  seems  to  me  to  guide  the  course 
of  a  mightier  current  upon  which  mechanical  causation  and  other 
influences  are  ripples  or  eddies,  or  at  least  no  more  than  the  waves 
whose  only  lasting  influence  is  occasionally  to  open  new  directions 
for  the  current  to  move  in.  With  this  belief  in  adaptation,  I  have 
naturally  not  hesitated  to  use  the  corresponding  language  of 
purpose, — not  a  mystical,  supernatural,  forethoughtful  purpose, 
but  a  physical,  natural,  experiential  purpose,  which  does  not 
presuppose  any  forethought,  but  only  the  preservation  and 
accumulation  of  the  results  of  past  experiences  wherein  each  step 
in  advance  was  purely  chanceful,  and  survived  only  because  it 
happened  to  fit. 

There  is  one  other  matter  of  this  kind  I  would  mention,  and 
that  will  be  all.  Throughout  the  book  I  have  made  great  use  of 
diagrams,  generalizations,  and  conventionalizations;  and  this  may 
seem  inconsistent  with  the  vitalistic  rather  than  mechanistic  tone 
of  the  work.  The  scientific  and  educational  status  of  this  practice 
are  sufficiently  explained  in  Chapter  I,  but  I  would  like  also  to 
say  that  I  think  our  advance  in  plant  physiology  is  measured 
exactly  by  our  ability  to  represent  each  detail  in  a  mechanical 
diagram,  a  physical  formula,  or  a  chemical  equation.  For  the 
evidence  certainly  indicates  that  every  individual  process  of 
plants  is  purely  mechanical,  physical,  or  chemical.  What  cannot 
thus  be  explained,  and  what  we  have  made  as  yet  little  progress 
towards  explaining,  is  the  nature  of  the  influence  which  establishes 
and  holds  these  processes  in  orderly  sequences  repeated  in  wonder- 
fully complicated  cycles  generation  after  generation.  When  we 
have  explained  the  operation  of  each  gun,  and  dynamo,  and 
powder-hoist  on  a  battleship,  have  we  thereby  explained  the 
rationale  of  the  operation  of  a  battleship?  Here  is  where  the  real 
difference  lies  today  between  mechanism  and  vitalism.  And  this 
is  the  vitalism  of  this  book, — not  a  supernatural  vitalism  of  the 
theological  type,  and  certainly  not  designed  for  theological  needs, 
but  a  perfectly  natural  vitalism  based  on  the  superior  interpretive 


viii  Preface 

power  of  an  hypothesis  assuming  the  existence  in  Nature  of  an 
X-entity,  additional  to  matter  and  energy  but  of  the  same  cosmic 
rank  as  they,  and  manifesting  itself  to  our  senses  only  through 
its  power  to  keep  a  certain  quantity  of  matter  and  energy  in  the 
continuous  orderly  ferment  we  call  life.  If  those  complicated  and 
regularly-recurring  cycles  of  material  and  energy  changes  which 
constitute  the  visible  phenomena  of  life  were  mechanistically 
self-originating,  self-controlling,  and  self-surviving,  then  Nature 
should  be  full  of  scattered  fragments  of  such  cycles,  whereas  she 
is  not.  For  everything  in  Nature  has  either  all  of  the  characteris- 
tics of  life,  or  else  it  has  none  of  them;  it  is  either  alive,  or  it  is  not. 
And  there  you  have  the  chief  argument  of  vitalism  against 
mechanism. 

Having  thus  explained,  the  best  that  I  can,  the  spirit  and  scope 
of  this  book,  I  turn  to  make  my  grateful  acknowledgement  to 
those  who  have  rendered  kind  aid  in  its  preparation.  For  the 
illustrations,  in  particular,  I  am  indebted  to  many  persons.  For 
the  privilege  of  using  the  two  dozen  or  more  fine  pictures  from 
Gray's  Structural  Botany  and  the  Chicago  Textbook,  as  acknowl- 
edged with  the  cuts,  I  am  indebted  to  the  publishers  of  those 
works,  the  American  Book  Company;  and  I  have  also  been  per- 
mitted by  the  Doubleday  Page  Company  to  use  figure  8,  and  by 
the  Bullard  Company  to  use  figure  15,  from  publications  of  theirs. 
Further,  a  ready  consent  has  been  given  by  Professor  G.  F.  Atkin- 
son to  my  use  of  figure  118,  and  by  Dr.  C.  C.  Curtis,  to  my  use  of 
figures  67  and  73,  from  books  of  theirs  published  by  Messrs.  Henry 
Holt  and  Company.  In  addition,  I  have  copied  a  number  of 
figures  from  various  foreign  works,  notably  those  of  Sachs, 
Kerner,  Strasburger  and  Kny,  taking  pains,  however,  to  acknowl- 
edge the  sources  with  the  cuts  themselves.  Further,  I  have  made 
use  without  special  acknowledgement  of  a  good  many  pictures 
which  have  been  copied  so  often  as  to  have  become  a  kind  of 
common  property  (viz.,  figures  17,  35,  94,  147,  149  to  161,  164. 
166-7,  169-171),  although  these,  together  with  certain  others 


Preface  ix 

whose  source  is  acknowledged  (viz.,  figures  81,  85,  107,  168,  175), 
have  been  re-drawn  for  this  work  by  one  of  my  students,  Miss 
Bertha  Bodwell,  now  Mrs.  Richard  Potter.  The  remainder  of  the 
pictures,  somewhat  over  one-half  of  those^in  the  book,  are  new. 
Several  have  been  made  by  students  of  mine: — figures  18  to  23, 
with  76  and  84  by  Miss  Bodwell:  figures  27,  56,  57,  132,  illustrat- 
ing physiological  apparatus,  with  126-7-8,  showing  phases  of 
growth,  by  Miss  Margaret  Sargent:  figures  103,  104,  parts  of  a 
series  representing  the  development  of  representative  plants,  by 
Miss  Ruth  Huntington,  now  Mrs.  Max  Brodel:  figure  87  by  Miss 
Stella  Streeter:  figure  133  by  Miss  Hope  Sherman:  while  the  fine 
graphs  of  figures  70  and  123  were  worked  out  from  the  original 
materials  as  well  as  drawn  by  Miss  Marion  Pleasants.  The  photo- 
graph of  figure  26  was  given  me  by  another  student,  Miss  Anne 
Barrows,  now  Mrs.  Walter  Seelye.  The  elaborate  and  exact 
drawing  of  root  tissues  forming  figure  53  was  made  by  my  col- 
league, Dr.  F.  Grace  Smith,  Associate  Professor  of  Botany  in 
Smith  College,  while  the  markedly  original  and  very  satisfactory 
series  of  generalized  drawings  in  illustration  of  the  principal 
physiological  processes,  embodied  on  the  colored  Plate  I,  and  in 
the  multiple  figures  54,  66,  139,  together  with  the  figures  30  and 
99,  were  specially  drawn  for  this  book  by  another  of  my  associates, 
Miss  Helen  A.  Choate,  Instructor  in  Botany  in  Smith  College. 
To  all  of  these  willing  and  efficient  collaborators  I  desire  here  to 
express  my  indebtedness,  and  my  grateful  thanks.  The  remainder 
of  the  illustrations,  including  the  new  photographs  and  diagrams, 
are  productions  of  my  own. 

But  the  greatest  of  my  obligations  is  to  Miss  Choate,  who  has 
read  both  manuscript  and  proofs  in  a  critical  spirit  no  less  militant 
because  friendly.  She  has  not  been  concerned  so  much  with  the 
scientific  aspects  of  the  chapters  as  with  their  exposition,  rep- 
resenting in  this  the  rights  of  the  reader,  for  whose  benefit  she 
has  curbed  much  exuberance  of  expression,  and  eliminated  mamr 
an  obscurity  and  inconsistency.  That  some  of  these  faults  re- 


x  Preface 

main  is  not  to  be  laid  to  her,  since  I  have  sometimes  leaned  back 
on  superior  official  authority  and  had  my  own  way. 

In  the  first  announcement  of  the  book  it  was  said  that  keys, 
similar  in  principle  to  those  used  in  works  on  classification,  would 
be  appended  as  aids  to  the  reader  in  finding  the  explanations  of 
phenomena.  These  keys,  however,  have  assumed  such  propor- 
tions that  it  seems  best  to  transfer  them  to  a  separate  work.  They 
are  now  in  process  of  elaboration  in  detail  by  another  of  my 
associates,  Miss  Julia  Paton,  Fellow  in  Botany  in  Smith  College, 
and  will  presently  appear  as  a  synoptical  handbook. 

Finally,  I  recall  that  in  advising  the  reader  to  try  as  many 
experiments  as  possible  for  himself,  I  said  that  practical  guides 
to  experimentation  would  be  suggested  in  the  Preface.  Un- 
fortunately the  one  of  these  I  consider  the  best,  I  am  forbidden 
by  modesty  to  name,  excepting  that  I  may  mention,  as  our  friend 
Mr.  Dooley  would  put  it  in  similar  case,  that  it  is  entitled  A 
Laboratory  Course  in  Plant  Physiology,  is  published  by  Messrs. 
Henry  Holt  and  Company,  and  is  written  by  myself. 

THE    AUTHOR. 
Smith  College, 

March  15,  1913. 


CONTENTS 


CHAPTER  PAGE 

I.  THE  VARIOUS  WAYS  IN  WHICH  PLANTS  APPEAL  TO  THE  INTERESTS 

AND  MIND  OF  MAN.    (Methods  of  Study  in  the  Science  of  Botany) . .       1 

II.    THE  PREVALENCE  OF  GREEN  COLOR  IN  PLANTS,  AND  THE  REASON  WHY 

IT  EXISTS.     (Chlorophyll  and  Photosynthesis) 16 

III.    THE   PROFOUND   EFFECT  ON  THE   STRUCTURE  OF  PLANTS  PRODUCED 

BY  THE  NEED  FOR  EXPOSURE  TO  LIGHT.    (Morphology  and  Ecology 

of  Leaves  and  Stems) 47 

IV.    THE  KINDS  OF  WORK  THAT  ARE  DONE  BY  PLANTS,  AND  THE  SOURCE 

OF  THEIR  POWER  TO  DO  IT.    (Respiration) 76 

V.  THE  VARIOUS  SUBSTANCES  MADE  BY  PLANTS,  AND  THE  USES  THEREOF 

TO  THEM  AND  TO  us.     (Metabolism) 105 

VI.    THE  SUBSTANCE  WHICH  IS  ALIVE  IN  PLANTS,  AND  ITS  MANY  REMARK- 
ABLE QUALITIES.     (Protoplasm) 138 

VII.    THE  WAYS  IN  WHICH  PLANTS  DRAW  INTO  THEMSELVES  THE  VARIOUS 

MATERIALS  THEY  NEED.     (Absorption;  Roots) 165 

VIII.    THE    WAYS    IN    WHICH    SUBSTANCES    ARE    TRANSPORTED    THROUGH 

PLANTS,  AND  FINALLY  REMOVED  THEREFROM.    (Transfer,  Trans- 
piration,  Excretion) 198 

IX.    THE    PECULIAR    POWER    POSSESSED    BY    PLANTS    TO    ADJUST    THEIR 
INDIVIDUAL   PARTS   TO   THEIR  IMMEDIATE    SURROUNDINGS.      (7m- 

tability) 224 

X.  THE  VARIOUS  WAYS  IN  WHICH  PLANTS  RESIST  THE  HOSTILE  FORCES 

AROUND  THEM.     (Protection.) 256 

XL  THE  WAYS  IN  WHICH   PLANTS  PERPETUATE   THEIR  KINDS,   AND 

MULTIPLY  THEMSELVES  IN  NUMBER.    (Reproduction) 278 

XII.  THE  MANY  REMARKABLE  ARRANGEMENTS  BY  WHICH  PLANTS  SECURE 

UNION  OF  THE  SEXES.     (Cross-pollination;  Flowers) 303 

XIII.  THE  WAYS  IN  WHICH  PLANTS  INCREASE  IN  SIZE,   AND  FORM  THEIR 

VARIOUS  PARTS.    (Growth;  physiological) 327 

XIV.  THE  ORDERLY  CYCLES  PURSUED  IN  GROWTH,  AND  THE  REMARKABLE 

RESULTS  OF  DISTURBANCE  THEREOF.    (Growth;  structural) 352 

XV.    THE  MANY  REMARKABLE  ARRANGEMENTS  BY  WHICH  PLANTS  SECURE 

CHANGE  OF  LOCATION.     (Dissemination;  Fruits) 378 

xi 


xii  Contents 

CHAPTER  PAGE 

XVI.   THE  METHOD  OF  ORIGIN  OF  NEW  SPECIES  AND  STRUCTURES,  AND  THE 
CAUSES  OF  THEIR  FITNESS  TO  THE  PLACES  THEY  LIVE  IN.   (Evolution 

and  Adaptation) 403 

XVII.    THE  REMARKABLE  IMPROVEMENT  MADE  IN  PLANTS  BY  MAN,  AND  THE 

WAY  HE  BRINGS  IT  ABOUT.    (Plant  breeding) 426 

XVIII.    THE    PRINCIPAL    GROUPS    INTO    WHICH    PLANTS    NATURALLY    FALL, 

WHETHER  BY  RELATIONSHIP  OR  HABIT.      (Classification) 445 

INDEX.  .  . .   467 


A  TABLE  DESIGNED  TO  Dtt 


The  description  and 
interpretation  of 
the  Living  Plant 
involves  consid- 
eration of, — 


The  interests  and 
capacity  of  the 
human  mind  in 
relation  to  the 
study  of  Plant 
Life,  discussed  in 
Chapter 

The  nature  and 
properties  of  liv- 
ing substance, 
called  Proto- 
plasm, of  plants, 
which,  however, 
can  be  under- 
stood better  after 
some  study  of 
the  physiological 
processes,  and 
hence  is  discussed 
in  Chapter  6. 
Protoplasm. 


The  physiological  proc- 
esses of  plants,  con- 
cerned with,— 


Maintenance  of  the 
Individual,  de- 
pendent on, — 


Nutritio 
vision 
phys 
requir 


Preservation  of  the 
Race,  dependent 
on, — 


The  methods  by 
which  plants  be- 
come altered  in 
structure,  habits, 
and  identity,  in- 
cluding,— 


The  methods  of  altei- 
ation 


Fitness 
roum 
quirin 


Replacei 
indivic 
quirinf 


Attainme 
size  b] 
viduali 
ing, — 

In  Natui 
Under  tt 


The  results  attained,  considered 


AY  THE  PLAN  OF  THIS  BOOK 


The  acquisition  of  food,  which  is  constructed  by 
plants  inside  their  own  tissues,  as  described  in 
Chapter 

The  development  of  photosynthetic  structures, 
to  which  is  devoted  Chapter 

The  release  of  energy,  which  supplies  the  power 
indispensable  for  every  kind  of  work,  as  shown 
in  Chapter 

The  transformation  of  food  into  special  sub- 
stances needed  for  particular  functions,  as  de- 
scribed in  Chapter 

[A  suitable  place  for  the  chapter  which  is  logi- 
cally No.  2,  as  noted  in  column  2] 

The  absorption  of  substances  into  the  plant, 
with  development  of  absorptive  structures; 
hence  Chapter 

The  movement  and  removal  of  substances 
through  and  out  of  plants,  considered  in 

[  Chapter 

[  The  adjustment  of  individual  parts  to  surround- 
ings, to  which  is  devoted  Chapter 

•j  The  development  of  protective  adaptations 
against  hostile  external  conditions,  discussed 

(  in  Chapter 

[  The  formation  and  development  of  new  indi- 
viduals like  those  which  produce  them;  hence 
Chapter 

The  development  of  sex-uniting  adaptations,  se- 
curing the  cooperation  of  two  parents  in  pro- 
duction of  offspring;  Chapter 

The  formation  of  new  parts  and  their  increase 
in  size,  to  which  is  devoted  Chapter 

The  development  of  structures  through  cycles, 
both  ontogenetic  and  climatic, — Chapter 

The  development  of  dispersive  adaptations,  se- 


curing room  for  new  individuals  to  grow,  as 

I      described  in  Chapter 

o  which  is  devoted  Chapter 16. 


Chapters 

1.  Methods      of 

Study 

2.  Photosyn 

thesis 

3.  Leaves      and 

Stems 

4.  Respiration 


5.  Metabolism 


6.  Protoplasm 

7.  Absorption; 

Roots 

8.  Transfer  and 

Excretion 


10. 


11. 


12. 


13. 


14. 


15. 


Irritability 
Protection 

Reproduc- 
tion 

Cross-pol- 
lination; 
Flowers 

Growth,     phy- 
siological 

Growth, 
structural 

Dissemina- 
tion; Fruits 

Evolution 


tand  of  man,  to  which  is  devoted  Chapter  .... 
hapter 


. .  17.  Plant  Breed- 
ing 

. .  18.  Classification 


THE  LIVING  PLANT 

CHAPTER  I 

THE  VARIOUS  WAYS  IN  WHICH  PLANTS  APPEAL  TO  THE 
INTERESTS  AND  MIND  OF  MAN 

Methods  of  Study  in  the  Science  of  Botany 

ND  he  spake  of  trees,  from  the  cedar  tree  that  is  in 
Lebanon  even  unto  the  hyssop  that  springeth  out  of 
the  wall."  Thus  runs  the  record  of  the  first  botanical 
teacher,  reputed  also  the  wisest  of  men,  as  writ  in  the 
greatest  of  books.  And  from  the  days  of  King  Solomon  down 
to  our  own,  men  never  have  ceased  to  speak  and  learn  of  plants, 
until  now  the  circle  of  knowledge  has  long  been  too  vast  for  any 
one  mind  to  encompass.  To  us,  plants  embrace  not  alone  the 
cedar  and  the  hyssop,  but  the  fern,  the  moss,  the  lichen,  the  sea- 
weed, the  mushroom,  the  mold,  the  blight,  the  yeast,  and  the 
germ  of  disease  within  the  body  of  man.  And  it  is  not  alone  their 
forms,  their  uses,  and  their  habits  which  concern  us,  but  as  well 
the  minutest  details  of  their  internal  construction:  the  mean- 
ings of  their  resemblances  and  their  differences :  the  ways  of  their 
nutrition,  increase,  and  adjustment  to  their  surroundings:  the 
possibilities  of  their  development  to  greater  and  yet  undiscovered 
utilities:  and  in  truth  no  less  than  every  fact  which  the  intellect 
of  man  can  discover  about  them. 

The  field  of  botanical  study  is  therefore  not  simply  vast,  it  is 
practically  limitless, — in  this  respect  transcending  the  natural 
powers  of  man,  which  are  small.  Therefore,  while  every  school- 


2  The  Living  Plant 

boy  can  grasp  the  salient  facts  in  that  organized  knowledge  of 
plants  which  we  call  the  Science  of  Botany,  no  one  person  can 
actually  master  any  more  than  a  limited  portion  thereof,  es- 
pecially if  he  have  the  ambition  to  know  it  sufficiently  well  to 
aid  in  expanding  the  bounds  of  our  knowledge.  For  the  purpose 
of  specialized  study,  accordingly,  there  have  been  developed 
within  the  science  a  number  of  divisions  which  are  dependent 
on  the  nature  of  the  problems  presented,  and  therefore  on  the 
methods  employed  in  their  study.  The  divisions  are  these.  First 
is  Classification  (called  also  Systematic  Botany,  or  Taxonomy},  the 
oldest  and  most  fundamental  of  all,  and  doubtless  the  theme  of 
King  Solomon's  discourse.  It  establishes  the  relationships  of 
plants  to  one  another,  and  arranges  them  accordingly,  while 
describing  and  naming  them.  It  is  studied  through  exact  ob- 
servation and  comparison  of  the  external  parts  of  plants,  which 
can  be  kept  preserved  in  a  pressed  and  dried  condition  in  col- 
lections called  Herbaria,  while  its  results  are  embodied  not 
only  hi  great  monographs,  but  in  handbooks,  or  Manuals,  so 
arranged  as  to  enable  any  person  to  identify  plants  for  himself. 
Second  is  Morphology,  which  deals  with  the  parts,  or  structures, 
of  plants,  and  establishes  their  relationships  to  one  another  while 
describing  and  naming  them.  Morphology  is  very  much  the 
same  to  the  parts  of  plants  that  classification  is  to  plants  as  a 
whole.  The  name  in  the  past  has  been  associated  most  closely 
with  the  comparative  study  of  the  large  external  structures, — 
roots,  stems,  leaves,  flowers,  and  fruits, — and  then*  transforma- 
tions into  tendrils,  spines,  pitchers  and  the  like,  but  is  nowadays 
given  a  far  wider  extension;  while  special  names  describe  the 
phases  concerned  with  minute  or  internal  parts,  and  needing  the 
use  of  such  exact  and  delicate  instruments  as  the  microscope 
and  microtome, — Embryology  or  " life-history,"  for  the  develop- 
ment of  the  structures  hi  the  individual  plant,  Anatomy,  for 
the  cellular  construction,  and  Cytology  for  the  internal  struc- 
ture of  the  cells  themselves.  Third  is  Physiology,  a  word  which 


The  Various  Ways  in  Which  Plants  Appeal  3 

has  precisely  the  same  meaning  with  plants  as  with  animals, 
comprehending  the  study  of  those  functions  or  processes  by  which 
they  secure  the  maintenance  of  their  daily  lives  and  the  per- 
petuation of  their  kinds.  It  is  studied  chiefly  through  experiment 
by  aid  of  the  exact  methods  and  instruments  of  physics  and 
chemistry,  though  it  reaches  into  realms  which  those  sciences 
do  not  touch.  Fourth  is  Ecology,  youngest  of  the  divisions  of  the 
science,  and  greater  as  yet  in  promise  than  performance,  but 
nevertheless  of  the  very  first  interest  to  a  great  many  people. 
It  explains  the  adaptations  of  plants  and  their  parts,  that  is, 
the  ways  in  which  these  are  adjusted  to  the  conditions  of  the 
world  around,  involving  the  meanings  of  their  forms,  sizes, 
colors  and  the  like.  This  division  has  sometimes  been  called, 
and  still  is  by  some  Germans,  Biology;  but  that  word  should  be 
kept  for  its  legitimate  use  as  meaning  the  study  of  life  com- 
prehensively, and  therefore  equivalent  to  Zoology  and  Botany 
together.  Fifth  is  Plant  Industry  (called  also  Economic  Botany), 
which  is  the  study  of  the  ways  in  which  plants  may  be  made  to 
yield  the  greatest  service  to  man.  The  older  phases  thereof, 
Agriculture,  Horticulture,  Pharmacology,  and  Forestry,  originally 
purely  practical,  are  now  scientifically  studied,  and  to  their 
very  great  profit;  while  strictly  scientific  from  their  foundation 
have  been  the  newer  phases  of  Pathology,  or  the  study  of  diseases, 
Bacteriology,  or  the  study  of  germs  and  their  effects,  and  Plant- 
breeding,  or  the  systematic  development  of  better  kinds  of  plants. 
And  to  these  divisions  there  is  every  promise  that  the  near  future 
will  add  yet  a  sixth,  Botanical  Education,  which  will  attempt  not 
only  to  train  students  much  better  in  the  science,  but  also  to 
interpret  botanical  progress  to  the  world  at  large.  An  important 
phase  of  this  division  will  be  the  production  of  works,  on  the 
Natural  History  of  Plants,  which  will  set  forth,  with  a  combination 
of  scientific  accuracy  and  literary  charm,  not  only  the  technical 
and  economic  aspects  of  plant  life,  but  also  those  historical, 
legendary,  and  imaginative  aspects  which  give  to  a  study  its 


4  The  Living  Plant 

widest  human  interest.  Indeed,  the  production  of  such  works 
may  be  viewed  as  the  logical  aim  of  all  botanical  study. 

Such  are  the  principal  divisions  of  botanical  science  as  we 
know  them  at  present.  This  book,  concerned  as  it  is  with  the 
life  of  plants,  deals  chiefly  with  Physiology,  but  the  divisions 
are  interlocked  inextricably,  and  I  must  perforce  make  many 
an  excursion  into  the  others.  This  science,  and  all  science,  is  a 
unit,  and  subdivisions  thereof  are  nothing  other  than  a  concession 
to  the  limitations  of  the  powers  of  man. 

As  the  reader  reflects  on  this  matter  of  the  various  divisions  of 
botanical  science,  he  cannot  but  notice  how  unequal  they  are  in 
apparent  utility  to  man,  and  he  may  even  inquire  why  we  should 
study  at  all  the  ones  that  seem  useless.  Two  reasons  at  least 
exist  why  we  should,  and  do.  First,  some  people  take  pleasure 
therein,  precisely  as  do  others  in  art,  music,  and  literature.  No- 
body thinks  of  asking  what  use  these  latter  may  be,  the  value  of 
pure  pleasure  being  obvious  enough;  but  the  world  has  mostly 
yet  to  learn  to  extend  the  same  approbation  to  the  seemingly  use- 
less sciences.  Second,  the  history  of  human  progress  has  shown 
that  the  greatest  applications  of  science  to  the  useful  arts  have 
sprung  from  purely  scientific  investigations  of  a  non-useful  type. 
Nothing,  doubtless,  could  have  seemed  more  useless  to  cotem- 
porary  critics  than  the  studies  of  those  early  naturalists  who  de- 
lighted to  apply  the  new-made  microscope  to  the  investigation  of 
the  living  atoms  which  swarm  in  slime;  and  yet  from  these  very 
studies  has  come  our  knowledge  of  Bacteria,  and  our  power  to 
control  the  deadliest  diseases  that  scourge  mankind.  Likewise 
photography,  all  the  applications  of  electricity,  a  vast  range  of 
chemical  arts,  and  indeed  most  others  of  the  wonderful  applica- 
tions of  science  to  utility,  have  developed  incidentally  from  purely 
abstract  scientific  researches  made  without  any  regard  to  useful 
applications.  Furthermore,  it  is  quite  impossible  to  predict  at 
what  point  upon  the  general  surface  of  expanding  knowledge  the 
next  useful  discovery  will  spring  forth.  In  fact  there  is  no  natural 


The  Various  Ways  in  Which  Plants  Appeal  5 

boundary  between  useful  and  useless  knowledge;  they  are  one 
and  indivisible,  and  such  boundary  as  may  seem  to  exist  is  simply 
a  shadow  that  shifts  over  the  surface,  changing  with  times  and 
our  customs.  Accordingly,  the  only  possible  way  in  which  human- 
ity can  obtain  useful  results  from  science,  lies  through  the  en- 
couragement of  the  development  of  all  of  its  phases;  and  this 
may  be  done  with  the  assurance  that  now  and  then  some  useful 
applications  will  somewhere  appear,  and  pay  manyfold  for  it  all. 
And  this  is  precisely  the  reason,  moreover,  why  no  good  system  of 
education  can  confine  itself  to  teaching  useful  knowledge  alone. 
It  is  unfortunately  still  tme,  as  it  was  when  Stephen  Hales,  the 
founder  of  Plant  Physiology,  wrote  nearly  two  centuries  ago,  that 
pure  science  needs  protection  "from  the  reproaches  that  the  ig- 
norant are  apt  unreasonably  to  cast  on  researches  of  this  kind, 
notwithstanding  that  they  are  the  only  solid  and  rational  means 
whereby  we  may  ever  hope  to  make  any  real  advance  in  the 
knowledge  of  Nature."  When,  therefore,  the  reader  hears  anyone 
asking  what  is  the  use  of  this  or  that  phase  of  knowledge,  or  when 
he  sees  practical  men  showing  impatience  with  the  impractica- 
bility of  great  scholars  and  contempt  for  the  uselessness  of  their 
knowledge,  he  may  well  state  these  facts  by  way  of  courteous 
reproof.  And  he  may  even  add,  as  to  such  knowledge,  that 
those  who  pursue  it,  in  the  absence  of  the  material  rewards 
reaped  in  full  measure  by  practical  men,  deserve  no  less  tribute 
of  respect  and  approbation  than  is  accorded  by  common  consent 
to  those  whose  efforts  bring  them  personal  wealth.  Both  in  fact, 
though  in  different  ways,  are  contributing  to  the  welfare  and 
progress  of  humanity. 

I  have  spoken,  just  now,  of  the  pleasures  of  the  study  of  Botany, 
and  over  this  theme  I  would  linger  a  little.  It  is  true  of  all  science 
that  the  pleasures  of  its  study  lie  deep,  and  one  must  reach  far 
before  he  can  grasp  them.  It  is  not  as  with  literature,  for  ex- 
ample, which  makes  appeal  to  the  feelings,  that  lie  near  the  surface 
and  are  easy  to  touch;  for  science  appeals  chiefly  to  reason,  which 


6  The  Living  Plant 

lies  deeper  and  is  slower  of  action.  This  is  why  literature  is  en- 
joyed by  nearly  all  people  and  science  by  only  a  few,  and  why 
literary  reputations  can  be  made  in  youth  while  those  of  science 
are  mostly  attained  much  later  in  life.  Yet,  when  grasped,  the 
pleasures  of  science  are  no  less  keen  than  those  derived  from  any 
other  field  of  intellectual  endeavor,  and  I  have  even  fancied  that 
they  yield  an  especially  deep  and  lasting  satisfaction,  though  in 
this  perhaps  I  am  wrong.  There  can  be,  I  believe,  no  pleasure 
in  life  any  greater  than  that  which  comes  to  the  scientific  man  with 
the  moment  in  which  some  truth  heretofore  not  known  to  man- 
kind first  dawns  upon  him;  and  it  is  in  the  hope  of  such  moments 
of  exaltation  that  he  is  willing  to  undergo  toil,  poverty,  hardship, 
and  even  peril  of  life  itself.  The  charm  that  there  is  in  this  pur- 
suit of  truth  receives  many  illustrations  from  the  biographies  of 
eminent  scientific  investigators,  and  especially  from  their  familiar 
letters,  in  which  can  be  seen  more  clearly  than  elsewhere  the 
actual  workings  of  the  scientific  spirit.*  But  though  felt  to  the 

*  A  characteristic  example  is  furnished  by  the  following  letter  written  by  Charles 
Darwin  to  Asa  Gray, — the  eminent  American  Botanist. 

Down,  August  9  [1862]. 

My  dear  Gray, — It  is  late  at  night,  and  I  am  going  to  write  briefly,  and  of  course 
to  beg  a  favour. 

The  Mitchella  very  good,  but  pollen  apparently  equal-sized.  I  have  just  examined 
Hottonia,  grand  difference  in  pollen.  Echium  vulgare,  a  humbug,  merely  a  case  like 
Thymus.  But  I  am  almost  stark  staring  mad  over  Ly thrum;  if  I  can  prove  what  I 
fully  believe;  it  is  a  grand  case  of  TRIMORPHISM,  with  three  different  pollens  and  three 
stigmas;  I  have  castrated  and  fertilized  above  ninety  flowers,  trying  all  the  eighteen 
distinct  crosses  which  are  possible  within  the  limits  of  this  one  species!  I  cannot  ex- 
plain, but  I  feel  sure  you  would  think  it  a  grand  case.  I  have  been  writing  to  Botan- 
ists to  see  if  I  can  possibly  get  L.  hyssopifolia,  and  it  has  just  flashed  on  me  that  you 
might  have  Lythrum  in  North  America,  and  I  have  looked  to  your  Manual.  For 
the  love  of  heaven  have  a  look  at  some  of  your  species,  and  if  you  can  get  me  seed, 
do;  I  want  much  to  try  species  with  few  stamens,  if  they  are  dimorphic;  Nesaea  vert- 
icilMa  I  should  expect  to  be  trimorphic.  Seed!  Seed!  Seed!  I  should  rather  like 
seed  of  Mitchella.  But  oh,  Lythrum! 

Your  utterly  mad  friend, 

C.  DARWIN. 

[Life  and  Letters  of  Charles  Darwin,  New  York,  1888,  II,  475.] 


The  Various  Ways  in  Which  Plants  Appeal  7 

fullest  only  by  those  who  fare  the  farthest,  the  pleasures  of  science 
are  by  no  means  unknown  even  to  youthful  students;  and  I  have 
myself  experienced  in  the  past  and  have  since  noticed  in  others, 
a  keen  enjoyment  in  the  use  of  exact  scientific  methods  and  tools, 
a  great  satisfaction  in  the  acquisition  of  knowledge  that  one  feels 
to  be  solidly  grounded,  and  a  lasting  pleasure  in  an  understand- 
ing of  the  workings  of  the  greater  natural  phenomena.  But  while 
the  personal  and  aesthetic  elements  are  certainly  by  no  means 
absent  from  scientific  study,  as  indeed  the  accompanying  picture 
will  bear  witness,  the  student  must  realize  that  the  deepest 
pleasures  of  science  are  of  stern  and  spartan  sort,  somewhat 
like  those  felt  by  the  strong  man  when  he  rejoiceth  to  run  a 
race. 

We  must  return  for  a  moment  to  the  matter  of  the  unity  of 
botanical  science  in  order  to  consider  yet  another  concession, 
besides  its  artificial  divisions,  to  human  limitations.  This  unity 
of  the  science  is  of  course  but  a  reflection  of  the  unity  of  Nature, 
where  all  of  the  vast  number  of  facts  and  phenomena  intergrade 
and  interlock  without  any  real  boundaries.  Yet  the  mind  of 
man  is  so  made  that  it  can  grasp  only  definite  conceptions,  and 
not  many  of  these;  and  it  can  no  more  form  a  definite  image  of  the 
infinite  intergradation  of  phenomena  than  it  can  of  the  infinite 
largeness  of  space  or  the  infinite  smallness  of  the  sub-constitution 
of  matter.  Hence  it  is  necessary,  for  purposes  of  education  and 
exposition,  to  create  definite  images  out  of  indefinite  material. 
Take,  as  an  example,  the  subject  of  leaves.  Leaves  are  so  many, 
so  diverse,  so  intergradient,  that  no  learner  can  grasp  any  con- 
siderable proportion  of  the  facts  about  leaves  as  they  actually 
are.  The  substitute  therefor,  to  which  every  teacher  and  author 
is  obliged  to  resort,  is  a  subjective  conception  of  a  generalized 
or  average  leaf,  built  up  for  the  learner  from  observation  of  a 
number  of  actual  leaves;  or,  better,  it  is  a  composite  conception 
of  a  leaf  built  up  in  the  receptive  mind  of  the  learner  from  many 
observations  of  actual  leaves,  much  as  composite  photographs 


The  Various  Ways  in  Which  Plants  Appeal  9 

of  human  faces  are  built  up  from  exposures  of  many  actual  faces 
upon  the  sensitive  photographic  plate.  This  is  precisely  what  our 
Text-books  are  doing  when  they  devote  chapters  to  "The  Leaf," 
"The  Stem,"  and  the  like.  These  titles  do  not  represent  things, 
but  ideas;  there  are  leaves  in  Nature  but  no  such  thing  as  the  leaf. 
But  the  analogy  of  these  composite  conceptions  to  composite 
photographs  goes  yet  a  step  farther,  for,  just  as  a  real  face  is  oc- 
casionally seen  which  resembles  the  composite  face  of  the  photo- 
graph, so  an  actual  structure  or  phenomenon  is  sometimes  found 
which  is  like  our  mental  composite  of  its  kind.  Such  a  real  thing  is 
then  said  to  be  typical,  and  that  is  what  is  actually  meant  by  this 
word  in  science.  When,  however,  no  typical  representative  of  the 
composite  is  available,  we  are  still  not  without  resources;  for  it  is 
possible  to  give  exact  and  clear  definition  to  the  dim  and  elusive 
outlines  of  the  composite  itself  by  drawing  firm  sweeping  lines 
through  its  more  prominent  places, — a  process  which  constitutes 
generalization,  or  conventionalization.  When  the  data  concerned 
are  expressed  in  figures,  then  the  result  is  a  round-number  aver- 
age, or  conventional  constant;  when  they  are  expressed  in  pictures, 
the  results  are  generalized  drawings,  or,  if  simplified  to  mere  struc- 
tural aids  to  the  imagination,  diagrams;  when  they  are  expressed 
in  words,  the  results  are  generalizations,  or  verities,  the  "aphor- 
isms" of  Bacon.  Throughout  this  book,  in  accordance  with  its 
aim  to  interpret  plant  life  in  the  large,  I  have  made  great  use 
of  composite  conceptions,  typical  things,  conventional  constants, 
generalized  drawings,  diagrams  and  verities, — to  a  degree  which 
will  meet  with  much  disapprobation  from  my  scientific  colleagues. 
But  I  maintain  that  such  generalized  knowledge  of  plants  is  not 
only  infinitely  better  than  no  knowledge  at  all,  but  is  actually 
the  most  useful  kind,  as  it  is  the  only  practicable  kind,  for  the 
non-technical  learner,  whose  knowledge  in  other  departments 
of  learning, — in  geography,  history,  and  so  forth, — is  largely  of 
this  character.  .And  I  further  maintain  that  if  only  we  would 
make  greater  use  of  it,  along  with  its  logically-correlated  methods, 


I0  The  Living  Plant 

in  our  educational  system,  we  should  have  less  cause  to  complain 
of  the  comparatively  empty  condition  of  our  elective  science 
classrooms.  It  is  not  of  course  representative  of  the  methods 
whereby  scientific  investigation  is  successfully  pursued;  but  where 
else  in  human  affairs  do  we  insist  upon  teaching  all  people  the 
technical  methods  or  none?  In  large  measure,  Science,  in  order 
to  be  advanced,  must  be  dehumanized;  but  hi  order  to  be  used, 
it  must  be  humanized. 

The  fact  is,  the  human  mind  is  a  very  poor  instrument  for 
scientific  research,  for  which  it  was  never  developed.  Unless  all 
of  our  knowledge  is  at  fault,  the  mind  of  man  was  evolved  under 
stress  of  use  as  Ms  chief  weapon  in  the  struggle  for  physical  ex- 
istence; naturally,  therefore,  all  of  its  stronger  traits  are  fitted 
to  that  very  concrete  activity  rather  than  to  uses  of  an  abstract 
intellectual  sort.  Its  power  of  concentration  upon  a  single  aim, 
with  determination  to  achieve  it  by  any  means:  its  instinctive 
and  partizan  exaltation  of  its  own  case  and  minimization  of  its 
opponent's :  its  tendency  to  warp  all  testimony  to  its  own  credit : 
its  quick  defense  of  its  'own  caste  or  clan,  right  or  wrong,  with  its 
ready  submission  to  the  conventions  thereof  and  contempt  for 
everything  outside:  its  preference  for  keeping  to  beaten  and  safe 
paths  and  for  shunning  the  unknown,  which  it  peoples  with 
mysteries  and  evil  designs :  its  liking  for  following  the  most  assert- 
ive leaders  and  for  leaning  back  upon  their  authority; — all  of 
these  are  invaluable  traits  in  the  struggle  of  the  individuals  of  a 
social  community  for  existence,  but  they  form  a  very  bad  basis 
for  scientific  investigation,  which  requires  the  opposite  qualities 
of  disinterestedness,  impartiality,  and  the  judicial  weighing  of 
evidence  for  the  determination  of  the  exact  truth  without  any 
regard  to  its  effects  upon  persons,  interests  or  dogmas.  All  men 
have  the  primitive  self-centering  qualities  highly  developed;  and 
the  scientific  research  of  mankind  is  done  upon  a  small  residue 
of  the  opposite  qualities  which  a  few  of  them  happen  to  possess, 
and  which  even  in  them  are  not  so  much  natural  as  assiduously 


The  Various  Ways  in  Which  Plants  Appeal         n 

cultivated.    Is  it  any  wonder,  then,  that  scientific  progress  is  so 
slow,  so  laborious,  and  so  expensive? 

There  remains  one  other  phase  of  the  relation  existing  between 
Science  and  the  mind  of  Man,  which  is  so  fundamental  to  the 
subject  of  this  book  that  we  must  give  it  some  special  attention. 
It  concerns  the  apparent  purposefulness  of  many  biological 
phenomena,  as  expressed  especially  in  adaptation.  What,  then, 
is  this  adaptation,  with  which  the  writings  of  Darwin  have  made 
us  so  familiar?  It  is  any  feature,  whether  of  structure  or  action, 
which  brings  a  life  process  into  harmonious  relation  with  the  ex- 
ternal conditions  that  affect  it.  The  flatness  of  a  leaf  is  an  adapta- 
tion to  the  need  for  a  very  wide  spread  of  green  tissue  to  light, 
as  is  to  be  fully  explained  in  the  following  chapter.  The  colors, 
shapes,  sizes  and  peculiarities  of  form  in  flowers  are  chiefly  adapta- 
tions to  the  utilization  of  insects  in  the  transfer  of  pollen,  which 
is  an  indispensable  prerequisite  to  cross  fertilization,  as  will 
also  be  demonstrated  in  the  suitable  place.  And  other  cases 
are  known  without  number,  involving  not  only  single  features, 
but  often  the  cooperation  of  several.  Now  the  question  is  this, — 
in  what  way  has  this  remarkable  fitness  of  form  to  function,  of 
structure  to  use,  of  parts  to  environments  arisen?  It  was  form- 
erly supposed  that  these  adaptations  were  the  direct  work  of  the 
Creator, — the  ETERNAL,  IMMEASURABLE,  OMNISCIENT,  and  OM- 
NIPOTENT,— as  LinnaBus  grandly  characterizes  him  in  the  Systema 
Natures.  But  Darwin  gave  evidence,  in  The  Origin  of  Species, 
greatest  of  all  secular  books,  tending  to  show  that  they  arose 
by  a  gradual  process  of  evolution,  developing  in  causative  touch 
at  every  step  with  the  conditions  which  they  fit;  and  this  view 
has  long  appealed  as  satisfactory  to  most  biologists.  But  in 
our  own  day  it  is  becoming  somewhat  customary  to  attribute 
adaptations  rather  to  various  adventitious  origins,  and  to  explain 
their  persistence  merely  by  the  negative  supposition  that  they 
are  not  out  of  harmony  with  the  conditions  concerned.  In  a  book 
of  this  kind  it  is  needful  to  take  a  definite  position  on  this  subject, 


12  The  Living  Plant 

if  for  no  other  reason  than  this, — that  the  language  one  may  use 
is  concerned.  My  position  in  general  is  the  Darwinian  one, — 
that  adaptation  in  the  main  has  arisen  as  a  gradual  causative 
accompaniment  of  evolution.  Indeed,  such  a  causative,  or  histor- 
ical development  of  adaptation  appears  to  me  an  inseparable 
corollary  of  the  very  idea  of  evolution,  and  wholly  independent 
of  its  method, — whether  it  proceed  by  many  imperceptibly  small 
steps  as  Darwin  believed,  or  by  fewer  and  perceptible  ones,  as 
newer  evidence  seems  to  be  showing.  And  the  point  about  use 
of  language  is  this,  that  if  adaptation  is  a  causative  process, — 
the  feature  developing  in  causal  touch  with  the  conditions  con- 
cerned,— then  it  is  quite  suitable  and  correct  to  say  that  the  adap- 
tation exists  for  such-and-such  a  purpose;  and  I  do  not  hesitate  to 
use  such  expressions  in  this  book.  In  so  doing  I  am  in  the  very 
best  of  company,  for  Darwin  himself  continually  uses  the  language 
of  purpose,  or  teleology;  and  both  Huxley  and  Asa  Gray,  Darwin's 
devoted  friends  and  co-believers,  point  out  in  their  writings  that 
evolution  on  the  basis  of  Natural  Selection  places  teleology  on  a 
scientific  basis.*  This  fact  is  overlooked  in  our  day  by  many, 
who  think  it  scientific  to  avoid  teleological  or  purposeful  language 
as  though  it  were  a  plague.  Science,  indeed,  hath  her  fashions 
and  her  dogmas  no  less  than  other  fields  of  human  endeavor. 

A  chief  reason  for  the  occasional  denials  of  the  causative  origin 
of  adaptation  arises  from  reaction  against  the  over-importance, 
and  over-perfection,  so  often  attributed  to  it.  Adaptation  has 
often  been  claimed  on  the  scantiest  evidence  without  any  attempt 
at  proof.  At  its  best,  however,  adaptation  can  never  be  perfect, 
but  is  rather  a  general  or  generic  affair,  very  much  like  our  own 
adaptations  to  the  trades  or  professions  we  follow.  This  is  be- 
cause no  feature  of  structure  or  function  is  free  to  respond  to  one 
adaptive  need  alone,  but  has  to  compromise  with  other  consider- 

*  An  example  of  Darwin's  teleological  language  is  found  in  the  passage  from  one 
of  his  books  cited  on  page  234  of  this  volume.  As  to  his  establishment  of  teleology 
as  a  scientific  principle,  compare  his  Life  and  Letters,  New  York,  1888,  II,  430. 


The  Various  Ways  in  Which  Plants  Appeal          13 

ations  which  often  have  more  influence  than  adaptation  itself. 
Thus,  in  addition  to  the  principal  adaptation,  (such  for  example 
as  the  flatness  of  a  leaf  in  adaptation  to  the  need  for  spreading 
much  surface  to  the  light),  there  are  secondary  adaptive  needs, 
such  as  for  protection  against  dryness  or  other  hostile  influences. 
Further,  a  prominent  feature  may  not  be  adaptive,  but  incidental 
to  some  other  process,  as  in  autumn  coloration  of  foliage,  or  the 
mathematically-arranged  origins  of  leaves:  or  it  may  be  merely 
a  mechanical  effect,  like  the  drooping  of  old  branches  of  evergreen 
trees:  or  it  may  represent  an  individual  adjustment  to  one  feature 
of  the  surroundings,  like  the  bent-over  leaf-stalks  of  house  plants 
in  windows :  or  it  may  be  inherited  from  the  past  without  present 
significance,  as  in  the  compound  early  leaves  of  the  Boston  Ivy: 
or  it  may  represent  a  spontaneous  new  variation,  or  mutation, 
or  sport,  such  as  originate  new  garden  varieties  of  flowers,  leaves, 
or  fruits;  or  it  may  have  yet  other  meanings  of  minor  sort.  These 
cases  and  illustrations  will  all  be  further  explained  in  the  following 
pages,  and  I  merely  cite  them  to  show  that  not  all  features  of 
plants  are  adaptations,  while  all  adaptations  are  interwoven  more 
or  less  with  these  other  considerations,  the  actual  structure  being 
the  resultant  of  the  interaction  of  them  all.  The  matter  can  be 
expressed  in  this  way,  that  adaptation  can  never  fit  a  condition 
as  an  old  glove  fits  the  hand,  but  rather  as  a  cloak  fits  the  body. 
One  should  therefore  neither  expect  too  much  of  it  on  the  one 
hand,  nor  reject  it  altogether  on  the  other.  The  real  problem  is 
not  so  much  to  find  adaptations  as  to  separate  out  and  define 
the  various  factors  that  enter  into  the  combinations  of  which 
adaptation  is  only  a  part. 

One  other  important  phase  of  the  relations  existing  between  the 
human  mind  and  the  workings  of  organic  nature,  concerns  the 
question  as  to  whether  there  is  anything  in  living  beings  except 
physics  and  chemistry, — in  other  words  whether  they  are  mechan- 
ism only,  or  whether  the  mechanism  is  inspired  by  vitalism.  The 
evidence  seems  to  be  showing  clearly  enough  that  all  of  the  in- 


14  The  Living  Plant 

dividual  processes  of  plants  and  animals  are  purely  physical  or 
chemical,  with  no  trace  of  a  vital  force  in  the  old  sense.  Further- 
more, the  orderl}'  sequence  and  cooperation  of  these  processes 
is  largely  explained  by  their  linking  up  through  the  medium 
of  stimuli,  as  will  later  be  explained  hi  the  suitable  places  in  this 
book.  But  it  does  not  seem  to  me  probable  that  the  processes 
only  happen  to  be  thus  linked  up,  or  that  these  particular  Unk- 
ings are  merely  the  accidental  survivors  of  innumerable  ones  that 
happened  in  the  past.  Indeed,  the  most  reasonable  explanation 
of  the  phenomena  of  organic  nature  in  the  large  seems  to  me  this, 
that  all  of  the  life  processes  are  subordinate  to  some  influence 
which  is  using  living  matter  as  a  seat  for  its  operations.  Thus 
there  would  exist  in  nature  not  two,  but  three  working  entities, 
matter,  energy,  and  this  X-influence.  Perhaps  the  living  matter 
is  the  home  which  the  principle  of  intelligence  in  Nature  has 
built  for  its  residence.  This  is  something  more  than  vitalism, 
or  even  the  neo- vitalism  of  some  philosophers;  it  is  a  super- 
vitalism.  But  its  acceptance  harmonizes  some  of  the  greatest 
difficulties  hi  the  interpretation  of  Nature,  as  the  following  pages 
will  illustrate  in  the  suitable  places. 

Finally  there  remains  one  matter  which  I  wish  to  add  at  this 
place.  It  may  seem  to  the  reader,  as  it  will  to  some  of  my  col- 
leagues, that  in  laying  so  much  stress  as  I  do  upon  causative 
adaptation,  and  a  number  of  things  of  that  sort,  I  am  reading 
into  Nature  a  principle  closely  akin  to  intelligence.  If  I  seem 
to  do  this  it  is  because  that  is  my  intention.  I  believe  that  the 
evidence  now  accumulating  is  sufficient  to  show  that  the  same 
principle  which  actuates  intelligence  also  actuates  all  the  work- 
ings of  Nature;  or,  as  I  have  expressed  the  matter  on  a  later  page 
of  this  book,  all  living  matter  thinks,  though  only  the  portion 
thereof  which  enters  into  the  brain  of  man  is  aware  that  it  thinks. 
Our  intelligence  is  a  kind  of  epitomized  expression  of  the  prin- 
ciples underlying  the  operations  of  nature,  very  much  as  mathe- 
matics is  an  epitomized  expression  of  the  relations  of  number, 


The  Various  Ways  in  Which  Plants  Appeal         15 

or  as  the  daily  newspaper  is  an  epitomized  expression  of  the  doings 
of  civilization.  And  this  I  mean  not  as  a  metaphor,  but  as  a 
serious  scientific  hypothesis. 

This  discussion  of  adaptation  and  kindred  matters,  and  per- 
haps some  others  of  the  matters  contained  in  this  chapter,  will 
have  little  meaning,  I  know,  to  the  reader  who  may  be  making 
his  first  acquaintance  with  plant  life  through  this  book.  But  I 
venture  to  hope  that  the  case  will  be  different  after  he  has  made 
some  study  of  the  pages  which  follow.  Perhaps  I  should  earlier 
have  advised  him  to  read  this  chapter  the  last;  and  at  least  I  do 
now  suggest  that  he  read  it  again  after  he  has  finished  the  rest 
of  the  book. 


CHAPTER  II 

THE  PREVALENCE  OF  GREEN  COLOR  IN  PLANTS,  AND 
THE  REASON  WHY  IT  EXISTS 

Chlorophyll  and  Photosynthesis 

0  manifold  are  the  works  displayed  in  the  world  of 
living  plants,  that  to  one  who  seeks  some  tie  to  bind 
them  all  into  a  single  natural  group  they  seem  at  first  to 
present  only  an  endless  diversity.  They  do  in  fact 
exhibit  every  possible  gradation  and  variation;  in  size,  from  the 
stately  Sequoia  of  the  Sierras,  or  the  giant  Eucalyptus  of  Aus- 
tralia, towering  high  above  all  other  living  things  and  mighty  in 
girth,  down  to  the  humblest  weed  of  the  wayside;  in  form,  from 
the  graceful  tree  with  its  spray  of  twigs  and  myriad  leaves  to  the 
simplest  sea-born  plant  whose  life  is  wholly  encompassed  within  a 
miniature  globe :  in  color,  from  the  quiet  green  of  the  forest  to  the 
brilliant  hues  of  flowers,  sea-mosses,  or  mushrooms:  in  texture, 
from  the  ivory-hard  seeds  of  palms  to  the  jelly-soft  fronds  of 
some  seaweeds;  in  habit,  from  the  independent  life  of  the  mightiest 
trees  in  the  woods  to  the  parasitic  existence  of  a  deadly  germ  of 
disease  within  the  body  of  man.  Nowhere  among  these  features, 
nor  yet  among  any  others  that  we  know,  can  we  find  a  single 
one  which  applies  to  all  plants.  What  is  it  then  which  binds  all 
of  this  heterogeneous  assemblage  into  a  single  natural  group? 

Failing  to  find  any  one  feature  common  to  all  kinds  of  plants, 
a  scientifically-minded  inquirer  would  next  turn  to  ask  what 
feature  prevails  most  widely  among  them.  If  one  marshals 
before  his  mental  vision  all  of  the  great  groups,  from  the  flowering 
trees  to  the  microscopical  germs,  and  centers  observation  upon 

16 


The  Prevalence  of  Green  Color  in  Plants  17 

one  after  another,  it  gradually  becomes  plain  that  one  feature, 
and  only  one,  does  prevail  very  widely, — and  that  is  the  possession 
of  green  color.  Moreover,  a  deeper  study  by  aid  of  microscope  and 
experiment  shows  that  this  truth  is  more  nearly  universal  than 
appears  at  first  sight,  for  a  good  many  plants  that  display  other 
colors, — e.  g.,  the  red  foliage  plants  of  the  gardens  and  the  brown 
and  red  seaweeds, — prove  to  be  green  in  reality,  though  that 
color  is  masked  by  the  presence  of  the  others. 

But  although  the  green  color,  which  is  that  of  a  definite  sub- 
stance called  chlorophyll,  is  thus  very  wide  spread  among  plants, 
there  are  some,  nevertheless,  which  really  do  not  have  it.  Such 
are  the  mushrooms,  molds,  mildews,  yeasts  and  germs,  as  like- 
wise the  Ghost  Plant  (or  Indian  Pipe),  of  the  woods,  the  twining 
Dodder  of  the  fields,  and  a  few  others.  These  plants  are  mostly 
white  to  brown,  though  they  often  exhibit  very  brilliant  hues  of 
red,  yellow,  and  even  a  kind  of  a  green,  which,  however,  is  very 
different  in  shade  and  nature  from  chlorophyll.  All  of  these 
brighter  colors  are  easily  removable  by  chemical  means;  and  when 
that  is  done,  the  tissues  are  left  either  white  or  brown,  with  never 
a  trace  of  the  chlorophyll. 

There  are,  accordingly,  plants  which  really  are  green  and 
plants  which  really  are  not.  And  the  reader's  first  natural 
thought,  that  so  striking  a  difference  in  one  feature  is  probably 
linked  with  differences  in  others,  is  correct.  In  the  first  place, 
observation  at  once  shows  a  very  fundamental  difference  between 
the  two  kinds  in  habit,  for  all  of  those  lacking  the  chlorophyll 
are  dependent  for  their  food  upon  other  beings,  either  upon  liv- 
ing plants  or  animals,  (in  which  case  they  are  called  parasites), 
or  else  upon  then-  decaying  remains,  (when  they  are  called 
saprophytes).  In  sharp  contradistinction  stand  the  green  plants, 
practically  all  of  which  subsist  without  aid  from  other  living 
things,  thriving  upon  materials  which  they  take  from  the  air, 
the  soil  and  the  waters.  A  second  great  difference  consists  in 
this,  that  all  of  the  non-green  plants  are  small  and  of  humble 


i8  The  Living  Plant 

habit,  as  the  list  above  given  will  testify,  contenting  themselves 
with  the  odd  and  obscure  places  of  nature,  while  the  green  plants 
grow  grandly  in  stature  and  number,  possessing  the  earth.  And 
still  a  third  difference  exists,  less  likely  to  be  thought  of  but  no 
less  important  for  our  present  inquiry,  namely,  the  study  of 
classification  has  shown  that  the  non-green  plants,  for  the  most 
part  at  least,  are  descended  in  the  course  of  a  long  evolution 
from  green  ancestors,  and  therefore  have  been  green  in  the  past. 
Hence  we  are  brought  to  a  generalization  of  the  greatest  impor- 
tance, the  first  indeed  of  the  great  botanical  verities, — the  pos- 
session of  chlorophyll  is  a  well-nigh  universal  characteristic  of 
plants,  and  their  most  distinctive  feature. 

Such  is  the  notable  fact  concerning  the  occurrence  of  chloro- 
phyll in  nature.  Obviously  so  wide-spread  a  substance  must 
play  some  very  great  part  in  the  life  processes  of  plants,  and  it  is 
our  manifest  duty  to  determine  what  it  is.  In  any  such  study 
the  first  resort  of  the  biologist, — his  first  aid,  as  it  were,  to  his 
ignorance, — is  observation,  exact  and  interrogative  observation, 
of  so  much  as  the  eye  can  discover.  If,  now,  the  reader  will  look 
over,  from  this  point  of  view,  any  collection  of  plants  in  garden  or 
greenhouse,  drawing  meanwhile  on  his  memory  for  additional 
facts  from  his  own  experience,  he  will  find  these  things  to  be  true; — 
that  chlorophyll  is  not  omnipresent  in  those  plants  which  pos- 
sess it,  being  absent  from  their  roots  and  interior  parts  not  reached 
by  the  light:  that  even  in  lighted  parts  it  is  not  uniformly  dis- 
tributed, being  denser  in  the  better-lighted  places,  as  well  ex- 
emplified in  the  deeper  green  of  the  upper  as  contrasted  with  the 
lower  faces  of  leaves:  that  it  does  not  develop  at  all  in  leaves 
which  are  grown  out  of  the  light,  as  witness  the  colorless  sprouts 
of  potatoes  started  in  the  darkness  of  cellars,  or  the  grass  of  lawns 
accidentally  left  covered  in  spring:  that  it  vanishes  from  green 
parts  kept  away  some  time  from  the  light,  as  shown  in  the  blanch- 
ing of  celery  when  banked  up  with  earth:  and  that  most  green 
parts  turn  over  towards  light  when  this  comes  rather  strongly 


PLATE    I 

Generalized  drawings  illustrating  the 
chlorophyll  system  of  the   plant 


PLATE     I 


C.    Single  cell  from  B.     D.    Single  chlorophyll 
grain  from  C. 


The  Prevalence  of  Green  Color  in  Plants  19 

from  one  side,  as  all  plants  kept  in  house  windows  attest.  All 
of  these  facts  unite  to  imply  an  extremely  close  relation  between 
the  meaning  of  chlorophyll  to  the  plant  and  the  action  of  light, 
even  suggesting,  indeed,  that  the  chlorophyll  is  inserted,  as  it 
were,,  between  the  light  and  the  use  thereof  by  the  plant.  To 
this  subject  we  shall  later  return,  for  we  are  dealing  at  present 
with  the  distribution  of  chlorophyll  in  the  individual  plant,  a 
matter  which  can  further  be  illustrated,  in  purely  diagrammatic 
or  conventional  fashion,  by  the  picture  which  forms  figure  A  of 
Plate  I  of  this  book.* 

So  important  is  chlorophyll,  that  the  reader  ought  really  to 
make  its  closer  acquaintance  through  actual  experiment ;  for  here, 
as  everywhere  else  in  science,  an  actual  personal  contact  with 
facts  or  phenomena  makes  all  the  difference  in  the  world  in  the 
clearness  of  one's  understanding  of  them.  It  is  possible  to  ex- 
tract the  chlorophyll  very  easily  from  leaves.  If  one  takes  two 
or  three  soft  thin  green  leaves,  places  them  in  any  glass  dish  which 
is  uninjured  by  heat,  covers  them  with  alcohol  (of  any  of  the  com- 
mon kinds),  and  lowers  the  dish  into  hot  water,  then  the  chloro- 
phyll will  come  out  into  the  alcohol  before  one's  very  eyes.  Its 
most  striking  characteristic  is  the  beautiful  green  color  of  the  clear 
solution,  together  with  a  remarkable  and  beautiful  red  fluorescence 
which  appears  when  the  solution  is  held  in  some  lights,  and  es- 
pecially when  sunlight  is  focussed  upon  it  with  a  lens.  And  the 

*  This  picture  is  meant  to  represent  that  which  one  would  see  on  a  surface  ex- 
posed by  a  lengthwise  cut  through  the  center  of  such  a  reduced  conventionalized 
plant.  Such  sections,  called  optical  sections,  are  very  much  used  in  biological  works. 
Thus,  on  the  very  same  plate,  (Plate  I),  appear  optical  sections  of  a  piece  of  a  leaf, 
a  single  cell,  and  a  chlorophyll  grain;  and  a  good  many  others  occur  elsewhere  in 
this  book.  In  every  case  an  optical  section  is  supposed  to  be  typical,  that  is,  taken 
through  the  part  most  illustrative  of  the  structure  in  question;  and,  where  only  one 
section  of  an  object  is  given,  it  means  that  the  object  is  substantially  alike  all  around 
the  axis  that  is  represented.  Such  sections,  therefore,  always  stand  for  solid  objects, 
and  the  reader  should  learn,  as  quickly  as  possible,  to  construct  the  solid  in  his  mind 
from  the  section  on  the  paper.  This  intellectual  visualization,  of  course,  requires 
imagination,  but  that  is  a  quality  which,  despite  the  popular  belief  to  the  contrary, 
is  highly  essential  to  success  in  science. 


20  The  Living  Plant 

reader  should  experiment  also  upon  its  instability  in  sunlight,  a  fact 
of  importance  as  will  later  be  proven;  this  he  may  do  by  dividing 
his  solution  into  two  portions,  of  which  he  puts  one  in  bright 
sunlight  and  awaits  its  changes  of  color,  while  he  places  the 
other  in  darkness  for  comparison.  Incidentally,  too,  this  experi- 
ment will  show  an  important  fact  about  the  color  of  leaves  apart 
from  their  coloring  matters,  for,  when  the  action  of  the  alcohol 
is  complete,  the  leaves  appear  a  soft  creamy  white.  This,  in 
fact,  is  the  natural  color  of  all  living  plant  tissues  when  no  special 
coloring  material  is  present. 

We  must,  however,  pursue  a  bit  farther  the  study  of  the  chloro- 
phyll substance,  partly  because  of  its  importance,  and  partly 
because  the  study  will  lead  the  reader  to  an  acquaintance  with 
other  matters  which  he  should  learn  very  early  in  his  botanical 
studies.  To  the  naked  eye  alone,  no  matter  how  closely  applied, 
the  chlorophyll  seems  to  color  uniformly  the  whole  of  the  leaf, 
which,  except  for  the  veins,  looks  homogeneous  in  texture.  But  if 
we  call  to  aid  that  wonderful  instrument  by  which  the  range  of  the 
eye  into  the  minute  is  increased  a  full  thousandfold, — that  first 
and  greatest  tool  of  the  biologist,  the  microscope, — and  place 
under  its  lenses  a  very  thin  section  or  slice  cut  right  through  some 
green  leaf  from  surface  to  surface,  then  a  very  different  idea  of 
leaf  structure  is  presented  to  the  observer,  as  the  accompanying 
picture  attests  (figure  2).  And  with  this  picture  of  an  actual  leaf, 
the  reader  should  compare  the  generalized  or  conventionalized 
section  represented  in  figure  B  on  Plate  I.  Clearly,  the  interior 
of  the  leaf  is  not  homogenous,  but  partitioned  into  a  great  many 
little  compartments,  with  empty  spaces  here  and  there  inter- 
spersed. These  compartments  are  called  cells,  a  word  of  vast 
importance  in  Biology,  because  not  only  the  leaf,  but  all  parts 
of  all  plants,  and  all  parts  of  all  animals,  are  composed  of  them. 
These  cells  differ  greatly  in  details  of  structure  according  to  their 
function,  but  are  always  compartments  of  some  sort;  and  the 
reader  should  as  promptly  as  possible  incorporate  this  idea  of 


The  Prevalence  of  Green  Color  in  Plants  21 

universal  cellular  structure  into  his  visual  conception  of  plants. 
In  our  picture  (figure  2),  carefully  drawn  from  an  actual  leaf,  and  as 
well  in  the  conventionalized  leaf  (B  on  Plate  I),  the  reader  can 
see  for  himself  the  cells  of  the  upper  and  lower  skin  (or  epidermis), 
those  of  the  vein  (the  clearer  mass  lacking  chlorophyll),  and 
finally  those  of  the  green  tissue,  distinguished  by  the  large  black 
or  green  spots  which  represent  the  chlorophyll  grains.  For  the 


FIG.  2.  A  thin  slice,  or  section,  cut  across  a  typical  leaf  (the  European  Beech),  and  highly 
magnified.  From  a  wall-chart  by  L.  Kny.  In  the  original,  the  numerous  black  discs 
are  green,  as  in  the  living  leaf. 


chlorophyll  really  is  contained  in  definite  grains,  and  is  not  a  dye 
spread  all  through  the  leaf.  These  cells  are  roughly  spherical, 
cylindrical,  or  polygonal  in  shape,  though  the  open  clear  air- 
spaces between  them  are  most  irregular  in  form.  Each  cell  has 
its  outer  thin  transparent  wall  (little  more  than  a  line  in  figure  2), 
within  which  comes  a  complete  lining  of  a  thin  gelatinous  sub- 
stance (shown  in  Plate  I,  B,  by  the  faint  grayish  or  dotted 


22  The  Living  Plant 

shading),  so  nearly  transparent  as  to  be  almost  invisible.  But 
though  so  insignificant  in  appearance,  this  grayish  material 
is  nevertheless  the  most  important  of  all  substances,  for  it  is  Proto- 
plasm, the  exclusive  seat  and  sole  physical  basis  of  all  the  phe- 
nomena of  life,  as  I  shall  show  in  a  later  chapter  devoted  to  that 
subject.  Within  this  living  substance,  close  up  to  the  wall,  lie 
the  chlorophyll  grains,  each  of  which  has  a  definite  shape,  some- 
thing like  that  of  a  disc  or  a  lens,  and  consists  of  denser  proto- 
plasm deeply  stained  by  a  green  liquid  which  is  the  chlorophyll 
substance  proper.  Finally,  it  should  be  added,  in  order  to  com- 
plete the  reader's  conception  of  the  cell,  that  all  of  the  remainder 
of  its  interior  is  filled  with  the  sap,  which  is  simply  water  contain- 
ing many  kinds  of  substances  in  solution.  As  to  the  spaces  be- 
tween the  cells,  they  contain  as  a  rule  nothing  but  air,  which  is 
in  connection  with  the  atmosphere  outside  of  the  plant  through 
tiny  little  openings,  called  stomata,  between  the  cells  of  the 
epidermis.  We  shall  return,  and  that  often,  to  this  subject  of 
cellular  structure,  and  the  reader  will  then  recognize  the  ad- 
vantage of  having  thus  made  some  preliminary  acquaintance 
therewith. 

We  must  now  return  to  the  problem  involved  in  the  observa- 
tion that  a  close  connection  exists  between  the  distribution  of 
chlorophyll  and  the  presence  of  light.  Observation  alone,  how- 
ever, cannot  lead  any  farther,  and  we  must  resort  to  the  second 
of  the  biologist's  methods, — experiment.  In  such  a  situation 
the  scientific  mind  would  reason  somewhat  like  this, — if,  as 
seems  implied  by  the  facts,  the  chlorophyll  has  in  the  plant  a 
function  dependent  on  the  action  of  light,  then  some  difference 
should  develop  between  leaves  kept  for  a  tune  in  darkness  and 
others  kept  equally  long  in  light.  Accordingly  the  experimenter 
would  darken  certain  leaves  on  a  plant,  in  a  way  that  would  not 
injure  their  health,  and  then,  after  a  day  or  two,  would  examine 
a  darkened  and  lighted  leaf  side  by  side.  The  result  is  always 
disappointing  to  the  naked  eye,  by  which  no  differences  at  all 


The  Prevalence  of  Green  Color  in  Plants  23 

are  discernible,  but  a  very  different  story  is  told  by  the  micro- 
scope. That  indispensable  instrument  shows  in  the  lighted  leaves 
the  presence  of  tiny  white  grains  (figure  D,  Plate  I),  which  are 
absent  from  the  leaves  that  were  darkened,  while  chemical  tests 
prove  these  grains  to  consist  of  a  definite  and  familiar  chemical 
substance, — starch. 

This  fact  that  starch  makes  appearance  in  ordinary  green 
leaves  when  exposed  to  the  light  but  not  in  those  kept  in  the  dark, 
is  so  important  in  plant  physiology  that  the  reader  should  make 
some  further  and  practical  acquaintance  with  the  matter.  If  he 
selects  some  one  of  the  commoner  house  plants,  (e.  g.,  Fuchsia, 
Garden  Nasturtium,  Horseshoe  Geranium),  covers  some  of  the 
leaves  from  the  light  by  a  box,  exposes  the  plant  for  a  day  or 
two  to  light,  removes  the  darkened  and  lighted  leaves  at  the 
close  of  the  second  day,  dips  them  for  a  moment  into  boiling  water, 
blanches  them  of  chlorophyll,  by  aid  of  warm  alcohol,  immerses 
them  in  water  a  minute  to  neutralize  the  brittleness  the  alcohol 
causes,  spreads  them  out  in  a  white  saucer,  and  covers  them  with 
a  solution  of  iodine  diluted  from  the  tincture  he  may  buy  from 
a  druggist,  he  will  be  rewarded  by  seeing  a  very  remarkable 
difference  develop  between  the  lighted  and  darkened  leaves,  for 
immediately  the  former  will  all  turn  a  very  dark  blue,  while  the 
latter  will  remain  of  their  natural  cream  color.  Now  iodine,  as 
anyone  may  prove  by  a  touch  to  some  part  of  his  starched  linen, 
though  brown  of  itself  turns  starch  a  dark  blue;  and  thus  our 
experiment  proves  that  the  leaves  form  starch  in  the  light  but 
not  in  the  dark.  So  exact,  indeed,  is  this  relation  that  if  a  famil- 
iar sharp  pattern  be  cut  in  opaque  material  and  applied  during 
the  experiment  to  the  upper  face  of  a  leaf,  that  pattern  is  found 
reproduced  in  equivalent  sharpness  when  the  iodine  test  is  ap- 
plied; and  not  only  this,  but  if  a  photographic  negative  be  used 
instead  of  the  pattern,  the  picture  will  be  printed  very  accurately 
in  starch  in  the  leaf,  and  may  be  " developed"  in  remarkable 
fashion  by  the  addition  of  iodine.  For  full  success  in  these  two 


24  The  Living  Plant 

latter  experiments,  however,  special  appliances  and  methods 
are  necessary;  and  these  are  fully  described  in  the  various  works 
devoted  to  experimental  plant  physiology,  and  mentioned  in  the 
preface  to  this  book. 

If  the  reader  should  experiment  at  all  widely  upon  this  matter 
of  starch  formation  in  leaves,  he  will  sooner  or  later  come  upon 
kinds  which  exhibit  no  starch  whatsoever,  even  under  perfect 
conditions  of  light.  Chemical  analysis,  however,  always  shows 
this  fact, — that  such  leaves  contain  an  equivalent  amount  of 
some  sugar.  Moreover,  and  this  is  a  matter  of  consequence, 
analysis  shows  also  that  even  the  starch-forming  leaves  contain 
a  sugar,  and  that,  furthermore,  it  is  from  this  same  sugar  the 
starch  is  made.  We  come  therefore  to  a  generalization  of  the 
greatest  physiological  consequence,  the  second,  in  fact,  of  the 
great  botanical  verities,  and  one  which  the  reader  should  fix  deep 
in  his  memory  and  incorporate  with  his  visualized  image  of  the 
working  green  plant,  that  plants  containing  chlorophyll  make  in 
the  light  a  sugar  which  is  commonly  transformed  into  starch.  The 
process  being  one  of  formation,  or  synthesis,  under  action  of 
light,  is  called  scientifically  photosynthesis,  while  the  substance 
made  is  the  photosynthate. 

It  will  sooner  or  later  occur  to  the  reader  to  ask,  especially 
if  he  has  tried  these  experiments  for  himself,  whether  this  photo- 
synthetic  sugar  is  simply  a  transformation  of  something  already 
existent  in  the  plant,  or  a  new  substance  that  has  been  added 
thereto.  This  can  be  settled  by  the  conclusive  test  of  compara- 
tive weights;  for,  obviously,  if  it  is  a  transformation,  photo- 
synthesis would  not  be  accompanied  by  increase  in  weight  while 
if  a  new  substance  it  would.  It  is  with  difficulty  that  I  resist 
the  temptation  to  describe  to  the  reader  the  simple  but  highly 
satisfactory  methods  and  instruments  by  which  this  important 
matter  is  experimentally  determined;  but  my  book  has  limits, 
and  besides  I  am  well  aware  that  any  attempt  to  exhaust  my  sub- 
ject is  likely  to  produce  a  similar  effect  on  my  reader.  So  I  must 


The  Prevalence  of  Green  Color  in  Plants  25 

simply  state  that  the  result  of  the  test  is  perfectly  conclusive, — it 
shows  that  leaves,  apart  from  varying  amounts  of  water  they  con- 
tain, always  gain  weight  in  the  light  but  not  in  the  dark.  They 
are  always  heavier  in  the  evening  than  they  were  in  the  morn- 
ing. As  to  what  becomes  of  the  starch  and  sugar  which  disappear 


This  square  is  Ja  of  a  meter  (a  decimeter)  on  a  side,  and  t J0  of  a  meter 
in  area. 

An  area  of  leaf  exactly  equal  to  this  square  would  make  iJo  of  a  gram 
of  grape  sugar  in  an  hour,  or  ^  of  a  gram  in  a  day,  or  1  gram  in 
10  days,  or  15  grams  (which  is  J  of  an  ounce)  in  a  summer. 

This  amount  of  grape  sugar  made  in  a  summer,  viz.  15  grams,  would 
form  a  cube  2.15  centimeters  on  a  side,  the  size  of  the  small 
square  in  the  lower  right  hand  corner  of  this  square.  Or,  it 
would  form  a  layer  over  this  entire  square  1  millimeter  (/B  of 
an  inch)  thick,  the  thickness  shown  by  the  space  between  the 
larger  and  smaller  squares. 


FIG.  3. — Diagram  to  illustrate  the  quantity  of  photosynthate  made  per  unit  area 
of  leaf. 

from  the  leaf,  that  will  later  be  shown,  though  we  may  here  note 
in  passing  that  there  is  a  continuous  movement  of  the  sugar  from 
the  leaves  into  the  stem.  Furthermore,  this  same  method  en- 
ables us  to  establish  the  amount  of  the  increase  in  weight.  This 
varies  greatly,  of  course,  with  different  plants  and  under  different 


26  The  Living  Plant 

conditions  of  light;  but  calculations  have  shown  that  for  many 
plants  collectively  out  of  doors  it  approximates  under  average 
summer  conditions  to  one  gram  for  each  square  meter  of  leaf 
area  per  hour  (scientifically  expressed  1  gm21i),  or  one  twenty- 
fifth  of  an  ounce  per  square  yard  per  hour,  and  is  about  half  that 


FIG.  4. — These  cubes,  which  are  two-fifths  the  original  size,  show  the  amount  of  solid 
crystalline  grape  sugar  made  by  a  square  meter  (or  yard)  of  leaf  in  an  hour,  a  day,  and 
a  summer. 

amount  in  greenhouse  plants  in  the  winter.  This  figure  con- 
stitutes one  of  those  useful  conventional  constants  which  the 
reader  should  store  in  his  mind,  and  keep  ready  for  use.  Ex- 
pressed in  a  different  way,  a  leaf  forms  in  a  summer  enough 
photosynthetic  sugar  to  cover  itself  with  a  solid  layer  a  millimeter 


The  Prevalence  of  Green  Color  in  Plants  27 

(one  twenty-fifth  of  an  inch)  thick.  The  same  quantities  are 
also  expressed  in  a  graphic  way  in  the  accompanying  figure  3,  and 
still  more  expressively,  perhaps,  in  figure  4. 

We  must  now  examine  more  closely  the  photosynthetic  sugar 
and  starch  which  appear  in  lighted  green  leaves.  The  microscope 
does  not  show  much  about  them,  for  the  sugar  is  always  dis- 
solved in  the  sap  of  the  cells,  and  the  starch,  although  solid,  is  in 
grains  too  small  to  be  seen  very  clearly.  Their  chemistry,  how- 
ever, is  well-known  and  important.  The  sugar  is  of  more  than 
one  kind,  but  the  commonest  is  that  known  as  grape  sugar, 
or  dextrose,  which  has  the  chemical  composition,  C6H12O6,  and 
which  is  intermixed  with  some  fruit  sugar  or  fructrose  having  an 
identical  formula.  This  formula,  I  need  hardly  say  to  the  reader 
of  this  book,  means  that  this  sugar  is  composed  of  6  parts  of 
carbon,  12  of  hydrogen  and  6  of  oxygen,  though  why  this  particu- 
lar combination  of  these  three  diverse  elements  should  give  a 
substance  with  the  properties  distinctive  of  grape  sugar,  nobody 
yet  knows.  Much  less  abundant  in  leaves  is  cane  sugar,  which 
has  the  composition  C12H220n.  Starch  has  for  its  formula 
(06Hi0O6)n,  the  n  meaning  a  multiple,  though  for  our  purposes 
we  may  treat  it  simply  as  C6H1005.  Now  it  is  immediately 
obvious  that  these  three  substances,  so  closely  associated  in  the 
leaves  of  plants,  are  also  very  closely  related  in  their  chemical 
composition,  for  they  differ  from  one  another  only  in  their  relative 
proportions  of  hydrogen  and  oxygen.  Thus, — 
C6H1206  —  H20  =  C6H1005 

grape  sugar      water          starch 

C12H22OU  +  H2O  =  2partsC6H1206 

cane  sugar        water  grape  sugar  and  fruit  sugar. 

C6H1005  +  H20  =  C6H1206 

starch  water       grape  sugar 

2  parts  C6H1206  —  H2O  =  C12H22OU 

grape  sugar        water        cane  sugar 

These  three  important  substances  thus  differ,  so  far  as  their 


28  The  Living  Plant 

composition  is  concerned,  simply  in  the  proportions  of  the  in- 
corporated water,  though  this  tells  by  no  means  all  of  the  story; 
but  it  helps  to  explain  why  they  are  so  easily  transformable  by 
the  plant  one  into  the  other.  Taken  together  the  facts  suggest 
the  probability  that  one  of  the  three  is  a  first-formed  or  basal 
substance  from  which  the  others  are  transformed.  In  a  general 
way  chemical  research  sustains  this  hypothesis,  and  points  to 
grape  sugar  as  the  usual  basal  substance  first  formed  in  the  light 
in  green  leaves.  For  all  of  our  purposes,  therefore,  we  may  accept 
grape  sugar  as  the  conventional  basal  photosynthate,  and  its 
formula  (C6  H12  06)  should  be  fixed  by  the  reader  in  his  memory 
as  another  of  the  valuable  conventional  constants. 

It  may  seem  to  the  reader  just  here  that  in  treating  this  sugar  so 
fully,  I  dwell  overlong  on  a  point  of  only  subordinate  value.  But 
in  this  my  critic  would  err,  for,  as  a  later  chapter  on  the  subject 
will  show  in  detail,  this  photosynthetic  grape  sugar  is  the  material 
from  which,  with  certain  transformations  and  some  additions, 
plants  make  all  of  their  substance  and  special  materials,  includ- 
ing their  protoplasm,  and  derive  all  of  their  energy  for  work;  in 
other  words,  it  is  their  food.  And  since  animals  all  take  their 
sustenance,  whether  directly  or  indirectly,  from  plants,  it  is  the 
basis  of  their  food  also.  These  facts  may  conveniently  be  brought 
together,  even  though  somewhat  in  advance  of  all  of  the  evidence, 
in  this  generalization,  which  constitutes  another  of  the  great 
botanical  verities, — that  the  photosynthetic  grape  sugar  formed 
in  green  leaves  in  the  light  is  the  basal  food  of  both  plants  and  ani- 
mals. This  sugar  is  therefore  one  of  the  three  most  important 
substances  in  organic  nature,  chlorophyll  and  protoplasm  being 
the  other  two. 

Our  next  task  is  sufficiently  obvious;  we  must  find  the  source  of 
supply  of  the  materials  entering  into  the  composition  of  the  sugar, 
which,  the  reader  will  remember,  is  an  addition  to  the  plant. 
Now  a  scrutiny,  from  this  point  of  view,  of  its  formula,  viz., 
C6H12O6,  at  once  reveals  the  suggestive  fact  that  the  H  and  the  O 


The  Prevalence  of  Green  Color  in  Plants  29 

are  present  in  exactly  the  proportions  they  exhibit  in  water, 
(H20) ;  this  suggests  that  they  may  be  derived  from  the  water 
which,  absorbed  from  the  soil,  always  saturates  the  tissues  of  the 
living  plant,  and  this  hypothesis  is  confirmed  by  experiment.  As 
to  the  carbon,  a  supply  thereof  exists  both  in  mineral  compounds 
in  the  soil,  and  also  in  the  carbon  dioxide,  commonly  called  car- 
bonic acid  gas,  in  the  atmosphere.  But  experiment  easily  de- 
cides between  these  two  sources,  for  when  plants  are  grown  in  a 
soil  or  in.  water  from  which  every  trace  of  carbon  is  excluded, 
the  plants  make  their  photosynthate  as  readily  as  ever,  thus  ap- 
parently proving  that  the  carbon  must  come  from  the  air.  At 
first  sight  it  may  seem  an  objection  that  this  gas  exists  in  the 
atmosphere  in  such  an  extreme  of  dilution,  for  it  comprises  only 
3  parts  in  10,000,  that  is  .03  (or  ^5)  of  1  per  cent.  This  amount 
is  very  small,  it  is  true,  though  we  must  remember  that  the  bulk 
of  the  whole  atmosphere  is  vast  in  proportion  to  the  bulk  of  all 
plants.  However,  suppositions  cut  small  figure  in  comparison 
with  facts;  and  it  is  easy  to  prove  by  simple  experiments  that 
leaves,  or  even  small  parts  thereof,  exposed  to  an  atmosphere 
from  which  the  carbon  dioxide  has  been  removed,  can  make  no 
starch  at  all,  although  neighboring  leaves  or  parts,  exposed  in 
the  ordinary  atmosphere,  form  it  abundantly.  Indeed,  innumer- 
able facts  unite  to  prove  that  the  carbon  used  by  leaves  hi  the 
making  of  sugar  is  derived  from  the  carbon  dioxide  (the  carbonic 
acid  gas),  of  the  atmosphere.  This,  as  the  reader  well  knows, 
is  the  very  same  gas  which  is  poured  out  by  animals  hi  breath- 
ing, by  organic  substances  in  decaying,  and  by  fires  in  burning. 
The  fact  that  leaves  absorb  this  gas  in  making  their  sugar  ex- 
plains in  part  the  scientific  basis  of  a  widely  known  and  very 
important  phenomenon, — that  plants  purify  the  air  which  is 
vitiated  by  animals. 

All  chemical  processes  can  be  expressed  in  equations  of  the 
formula?  of  the  substances  concerned,  and  therefore  we  proceed 
to  set  down  together  the  formula?  of  the  carbon  dioxide  (viz., 


30  The  Living  Plant 

CO2),  and  water,  with  the  formula  of  the  grape  sugar  they  form, 
thus, — 

In  photosynthesis  C02    and    H20  form  C6H1206 

carbon  dioxide        water  grape  sugar 

Obviously  now  the  proportions  of  the  two  former  must  be  in- 
creased in  order  to  yield  the  latter,  thus, — 

6  C02  +  6  H2O  are  needed  to  form  C6H1206 

But  a  chemical  equation  must  balance  exactly  on  the  two  sides, 
and  this  in  the  present  case  can  occur  only  thus, — 
6  CO2  +  6  H20  =  C6H12O6  +  6  O2 

But  such  a  balance  of  the  equation  implies  that,  in  the  making 
of  sugar  from  carbon  dioxide  and  water,  oxygen  is  set  free,  and 
not  only  so,  but  in  a  volume  exactly  equal  to  that  of  the  carbon 
dioxide  absorbed.  So  striking  a  conclusion  based  upon  purely 
theoretical  evidence  demands  rigid  test  through  observation  or 
experiment.  That  a  gas  of  some  kind  is  released  from  green 
plants  in  the  light  is  easily  seen  in  submerged  water  plants  which, 
if  kept  in  an  aquarium,  give  off  tiny  bubbles  when  lighted,  though 
not  in  the  dark;  and  everybody  has  seen  those  large  gas  bubbles 
which  are  caught  in  the  felted  green  scum-plants  floating  on  ponds. 
Analysis  shows  that  the  bubbles,  in  both  cases,  consist  mainly  of 
oxygen.  But  the  matter  can  be  tested  much  better  by  experiments. 
In  a  word,  it  is  only  necessary  to  place  a  green  plant  or  a  leaf  in 
a  suitable  tight  glass  chamber,  give  it  a  known  quantity  of  carbon 
dioxide  (it  has  plenty  of  water),  expose  it  for  some  time  to  the 
light,  and  then  make  a  chemical  analysis  of  the  air  in  the  chamber. 
The  experiment  yields  an  invariable  result.  A  certain  amount 
of  the  carbon  dioxide  has  disappeared,  and  in  its  place  there  is 
present  an  exactly  equivalent  amount  of  pure  oxygen.  As  to  the 
significance  thereof,  it  seems  plain  that  the  oxygen  is  a  by- 
product formed  incidentally  in  the  chemical  transformations,  and 
useless  hi  the  main  process. 


The  Prevalence  of  Green  Color  in  Plants  31 

Thus  is  our  equation  triumphantly  vindicated,  and  we  shall 
know  it  henceforth  as  the  photosynthetic  equation.  Its  importance 
and  meaning  may  thus  be  expressed  as  another  of  our  botanical 
verities,  —  that  the  photosynthetic  sugar  made  in  green  leaves  in 
light  is  constructed  from  water  drawn  from  the  soil,  and  carbon  di- 
oxide derived  from  the  atmosphere,  with  an  incidental  release  of  pure 
oxygen,  according  to  the  photosynthetic  equation  6  C03  +  6  H20  = 


It  may  interest  the  reader  now  to  know  what  quantities  of 
these  gases  are  necessary  in  the  making  of  the  sugar.  For  one 
gram  thereof  there  are  required  750  cubic  centimeters  (about 
f  of  a  quart)  of  pure  carbon  dioxide,  which  is  all  that  is  con- 
tained in  2  cubic  meters  of  atmosphere,  and  there  is  released  the 
same  quantity  of  pure  oxygen.  This,  therefore,  is  the  amount 
of  those  gases  absorbed  and  released  by  a  square  meter  (or  yard) 
of  green  leaf  each  hour  on  a  bright  summer  day.  This  release 
of  oxygen,  by  the  way,  explains  the  remainder  of  the  fact  earlier 
mentioned,  that  plants  purify  the  air  which  animals  vitiate,  for 
the  plants  not  only  remove  the  poisonous  carbon  dioxide  from  the 
air,  but  replace  it  by  pure  oxygen.  And  it  may  interest  the  reader 
to  know  how  this  balance  of  purification  and  vitiation  works  out 
between  green  leaves  and  men.  Calculations  have  shown,  in 
brief,  that  about  25  square  meters  (or  yards)  of  green  leaf  are  re- 
quired to  balance  the  respiration  of  a  man  on  an  ordinary  sum- 
mer day.  But  as  the  release  of  oxygen  stops  at  night,  it  takes 
about  60  square  meters  of  leaf  working  for  a  day  to  balance  the 
man's  respiration  for  24  hours,  and  about  150  square  meters  work- 
ing through  the  summer  to  balance  his  respiration  for  a  year. 

In  composing  the  foregoing  paragraphs  I  have  given  much  care 
to  the  form  of  their  presentation,  for  the  reason  that  this  particu- 
lar topic  illustrates  exceptionally  well  the  principal  method  of 
scientific  procedure  in  the  acquisition  of  new  knowledge.  First, 
in  the  given  problem,  to  observe  all  the  facts  that  the  militant 
eye  can  discover  :  next  to  compare  and  marshal  the  data  thus  won 


32  The  Living  Plant 

with  a  view  to  finding  an  explanatory  principle :  then  to  express  the 
most  probable  conclusion  in  tentative  form  as  an  hypothesis: 
and  finally  to  devise  experiments  whereby  the  truth  or  falsity 
of  the  hypothesis  may  be  tested;  these  are  the  constituents  of 
that  scientific  method  through  which  all  of  our  great  scientific 
triumphs  have  been  won.  Hypothesis  is  a  kind  of  a  scout  which 
Science  sends  on  ahead  to  spy  out  the  way  for  a  further  advance.* 
For  the  completion  of  our  subject  of  photosynthesis,  there  re- 
mains but  one  matter  of  consequence,  and  that  is  the  explanation 
of  the  association  of  light  and  chlorophyll  with  the  process.  We 
have  seen  earlier  that  the  chlorophyll  occupies  a  position  between 
the  light  and  the  new-made  starch  or  sugar,  which  fact  implies 
that  it  forms  a  necessary  link  between  the  two.  This  in  turn 
would  suggest  that  the  chlorophyll  perhaps  acts  on  the  light  in  a 
way  to  make  it  available  for  the  photosynthetic  process.  Tak- 
ing this  hypothesis  for  guidance,  we  turn  to  investigate  the  effect 
that  chlorophyll  exerts  upon  the  light  which  penetrates  into  it. 
Now  the  sunlight,  as  everybody  knows,  is  a  composite  mixture 
of  vari-colored  lights,  which,  taken  together,  give  the  impression 
of  whiteness.  If  this  sunlight,  however,  be  passed  through 
chlorophyll,  whether  a  living  leaf  or  a  solution  in  alcohol,  there 
issues,  as  the  reader  will  recall,  only  a  clear  green,  or  yellowish- 
green,  light;  and  this  fact  seems  to  imply  that  all  of  the  colors 

*  That  this  is  in  practice,  as  it  is  in  theory,  the  method  of  scientific  men  in  their 
researches  is  illustrated  by  the  following  passage  from  the  writings  of  the  great 
German  physiologist,  Sachs.  In  connection  with  this  very  subject  of  starch  forma- 
tion, he  tells  of  his  preliminary  observations,  on  the  basis  of  which,  he  says, — "  I 
came  to  the  conclusion  in  1862  that  the  enclosed  starch,  which  had  already  been  ob- 
served in  the  chlorophyll-corpuscles  by  Naegeli  and  Mohl,  is  to  be  regarded  as  the 
first  evident  product  of  assimilation  [i.  e.,  photosynthesis]  formed  by  the  decom- 
position of  carbon  dioxide.  I  said  to  myself,  if  this  view  is  right,  the  formation  of 
starch  in  the  chlorophyll-corpuscles  must  cease  on  the  exclusion  of  light,  since  the 
decomposition  of  carbon  dioxide  can  then  no  longer  take  place;  and  that  in  like  man- 
ner renewed  access  of  light  to  the  chlorophyll-corpuscles  must  also  bring  about  a 
renewal  of  the  formation  of  starch  in  them.  These  and  similar  deductions  were  con- 
firmed by  appropriate  investigations."  (Lectures  on  the  Physiology  of  Plants, 
Oxford,  1887,  p.  307.) 


The  Prevalence  of  Green  Color  in  Plants 


33 


in  sunlight  have  been  stopped  by  the  chlorophyll  excepting  only 
the  green.  But  the  human  eye  is  far  too  crude  an  analyzer  of 
color  to  be  scientifically  trustworthy,  and  we  turn  for  aid  to  an 
instrument  which  science  has  devised  for  the  exact  analysis  of 
light, — the  spectroscope.  I  confess,  it  is  only  with  reluctance  that 
I  refrain  from  explaining  to  the  reader  the  principle  of  this  beauti- 


FIG.  5. — Diagrams  to  illustrate  analysis  of  light  by  the  spectro- 
scope, a.  Spectrum  of  pure  sunlight.  6.  Spectrum  of  sun- 
light passed  through  chlorophyll. 


ful  instrument,  one  of  the  most  delicate  and  exact,  but  withal  one 
of  the  simplest  in  theory,  of  all  that  have  yet  been  evolved 
in  the  progress  of  science.  It  must  suffice  to  say  that  the  spectro- 
scope takes  any  mixture  of  colored  lights,  no  matter  in  what 
complication,  and,  through  the  mediation  of  a  prism,  spreads 
them  out  in  a  band  (called  a  spectrum),  each  color  by  itself.  So, 
when  a  ray  of  white  sunlight  is  sent  into  this  instrument,  it  is 
made  to  fringe  out  into  its  red,  orange,  yellow,  green,  blue,  in- 
digo and  violet  constituents,  all  beautifully  clear  and  distinct,  as 
shown  diagrammatically  in  our  accompanying  figure  5,  a.  Now 


34  The  Living  Plant 

if,  while  one  is  observing  this  spectrum,  a  solution  of  chlorophyll 
is  inserted  into  the  path  of  the  light,  a  remarkable  phenomenon 
follows,  for  the  green  liquid  blots  out  from  the  spectrum  most  of 
the  red  and  nearly  all  of  the  blue-indigo-violet,  making  those 
parts  of  the  spectrum  quite  black,  while  all  of  the  rest  of  the  colors 
are  left  practically  unaffected,  as  represented  in  our  diagram 
(figure  5,  6).  Chlorophyll,  therefore,  has  power  to  absorb  red 
and  blue  rays  out  of  the  sunlight,  ignoring  the  others, — in  ob- 
serving which  fact  the  active  scientific  mind  would  jump  straight 
to  the  conclusion  that  these  red  and  blue  rays  are  probably  the 
ones  which  are  useful  in  photosynthesis.  This  hypothesis  also 
is  easily  tested  by  experiment,  for,  obviously,  if  the  red  and  blue 
rays  really  are  those  used  in  photosynthesis,  while  the  others  are 
not,  then  starch  ought  to  be  made  under  red  light  and  blue  light, 
but  not  under  any  others  of  the  colors  of  the  spectrum.  It  is 
possible  to  supply  the  different  colored  lights  singly  to  the  green 
leaf,  either  by  use  of  colored  glasses  or  liquids  or  by  throwing  a 
solar  spectrum  directly  upon  a  leaf.  The  result  of  the  experiment 
is  conclusive;  a  leaf  can  form  starch  very  readily  under  red  light 
or  blue  light;  but  it  can  form  none  at  all  under  the  yellow,  orange, 
or  green.  It  seems  a  safe  inference,  therefore,  that  chlorophyll  is 
a  substance  which  picks  out  of  white  sunlight  and  applies  to 
photosynthetic  work,  just  those  rays  which  can  be  utilized  in  the 
photosynthetic  process,  while  rejecting  the  others;  and  all  evidence 
attests  the  correctness  of  this  conclusion. 

This  conclusion,  however,  raises  a  correlated  question,  which  is 
this, — for  what  particular  purpose  is  light  needed  in  photosyn- 
thesis? Light,  of  course,  is  a  form  of  energy,  like  heat  and  elec- 
tricity; and  energy  is  the  source  of  power  which  underlies  every 
kind  of  work.  Light,  so  physicists  teach,  consists  of  wave- 
motions  in  a  space-pervading  medium  called  the  luminiferous 
ether;  and  the  motion  of  these  ether  waves  forms  a  source  of 
power  that  can  accomplish  work,  just  as  surely  as  can  the  billows 
of  the  ocean.  Our  problem,  then,  resolves  itself  into  this, — is 


The  Prevalence  of  Green  Color  in  Plants  35 

there  in  photosynthesis  any  step  requiring  the  doing  of  work, 
and  therefore  the  expenditure  of  energy?  Our  photosynthetic 
equation  supplies  the  answer,  for  it  shows  that  the  oxygen  set 
free  has  to  be  torn  away  from  either  the  carbon  or  the  hydrogen 
of  the  carbon  dioxide  or  water,  as  a  necessary  preliminary  to  the 
union  of  the  carbon  with  the  remaining  elements  to  form  sugar; 
and  other  evidence  shows  that  the  carbon  dioxide  at  least 
is  thus  dissociated.  Now  carbon  dioxide  is  among  the  most 
stable  of  natural  compounds,  which  means  that  its  constituent 
atoms  have  an  extremely  strong  affinity  for  one  another,  which 
means  in  turn  that  ample  power  must  be  exerted  to  tear  them 
apart.  Most  people  know  that  in  our  laboratories  water  can  be 
separated  into  its  constituent  hydrogen  and  oxygen  only  through 
action  of  an  electric  current  (electrolysis),  or  of  intense  heat;  but 
carbon  dioxide  is  even  more  difficult  of  dissociation.  Here  then, 
in  the  preliminary  dissociation  of  this  very  refractory  substance 
is  that  need  for  energy  which  we  seek;  and  all  the  results  of  re- 
search confirm  this  conclusion.  Why  it  should  be  the  red  and 
blue  rays  and  no  others  which  can  do  this  work,  we  do  not  yet 
know,  nor  yet  precisely  the  way  in  which  the  chlorophyll  applies 
them  to  the  task;  but  there  is  no  question  as  to  the  facts.  That  is, 
chlorophyll  is  a  transformer  of  light  energy  into  photosynthetic 
work;  and  there  you  have  the  explanation  of  its  function  in 
plants,  and  the  reason  for  its  overwhelming  prevalence  in 
vegetation. 

We  can  now  summarize  this  part  of  our  subject  as  another  of 
our  botanical  verities, — the  formation  of  photosynthetic  sugar  in 
leaves  requires  first  the  dissociation  of  the  refractory  carbon  dioxide, 
which  is  effected  by  the  energy  of  the  red  and  blue  rays  of  the  sunlight, 
applied  to  that  work  by  the  chlorophyll. 

It  will  perhaps  contribute  further  to  clearness  if  we  summarize 
the  whole  process  of  photosynthesis  from  another,  and  very  human 
point  of  view.  The  formation  of  the  photosynthetic  sugar,  the 
end  of  the  whole  process,  is,  after  all,  a  manufacturing  process 


36  The  Living  Plant 

comparable  directly  with  those  carried  on  by  men,  as  the  fol- 
lowing table  well  shows. 

The  Factory  The  Leaf,  or  other  green  structure. 

Rooms  therein  The  cells. 

The  power  Sunlight,  the  red  and  blue  rays. 

The  machinery  Chlorophyll. 

The  raw  materials  Carbon  dioxide  and  water. 

The  manufactured  product  Grape  Sugar. 

By-products  Oxygen. 

The  photosynthetic  machinery  can  not  only  be  apprehended, 
but  also  represented  in  a  mechanical  plan,  as  our  accompanying 
diagram  illustrates  (figure  6).  It  represents  the  parts  concerned 
in  the  process,  (shown  simplified  in  figure  B  on  Plate  I,)  reduced 
each  to  a  single  one,  and  given  a  regular  shape,  though  otherwise 
constructed  and  related  as  hi  the  plant.  Later  we  shall  consider 
exactly  the  forces  which  keep  the  gases  and  liquids  in  motion 
in  the  suitable  directions. 

The  reader  should  now  be  able  to  visualize,  or  see  vividly  in 
imagination,  this  process  in  progress.  Streaming  in  through  the 
stomata  and  along  the  air  passages  is  a  steady  current  of  the  tiny 
particles,  or  molecules,  of  carbon  dioxide,  which  reach  the  cell 
walls,  pass  in  solution  through  these  and  the  protoplasm  into  the 
chlorophyll  grains,  where  they  meet  with  water  supplied  in  a 
continuous  stream  by  the  ducts.  Here  in  the  grain  the  chloro- 
phyll is  stopping  the  red  and  blue  light,  and  turning  then*  vi- 
brating waves  against  the  molecules  of  the  carbon  dioxide  in  a 
way  to  shatter  that  substance  into  its  constituent  atoms.  The 
carbon  thus  forced  apart  from  its  own  oxygen  is  uniting  with 
the  elements  of  the  water  into  sugar,  which  is  streaming  into  the 
sap  cavity  and  then  away  through  the  sieve  tubes,  while  the  dis- 
carded oxygen  is  passing  out  from  the  grains  through  protoplasm 
and  wall  to  the  air  space,  along  which  it  is  streaming  to  the 
stomata  and  the  outside  world.  And  this  is  what  is  occurring 
inside  of  all  leaves  through  all  the  bright  days  of  the  summer. 


The  Prevalence  of  Green  Color  in  Plants 


37 


So  striking  and  far-reaching  are  the  conclusions  already  reached 
in  this  chapter  that  anything  added  thereto  must  come  as  a  kind 
of  anti-climax;  and  therefore  I  wish  we  could  stop  just  here. 
Moreover  the  chapter  is  al- 
ready over-long,  though  no 
longer,  I  maintain,  than  the 
relative  importance  of  its 
subject  sufficiently  justifies, 
especially  as  it  seemed  to  me 
best  to  make  this  first  treat- 
ment of  very  important  top- 
ics illustrate  the  methods 
through  which  our  scientific 
knowledge  has  been  gained. 
Yet  several  closely  related 
matters,  especially  concern- 
ing the  colors  of  plants, 
should  have  our  attention 
before  we  depart  from  this 
subject,  though  I  venture  to 
suggest  to  the  reader  that  he 
should  not  attempt  to  read 
all  of  this  chapter  at  one 
sitting,  but  reserve  the  fol- 


„ 

FIG.  6.—  A  diagram  of  the  photosynthetic  ma- 
chinery.  showing  the  parts  reduced  to  the  low- 
est  possible  terms,  viz.,  a  single  living  cell,  with 
a  single  chlorophyll  grain,  a  water-carrying 
duct  (on  the  left>  and  a  sugar-carrying  sieve- 
tube  (on  the  right)  ;  shading  is  protoplasm. 
he  circles  are  water;  the  squares  are  carbon 
dioxide;  the  triangles  are  oxygen;  the  crosses 
are  grape  sugar;  the  arrows  show  the  direc- 
i  ,,  i  11  i  i  tions  of  movement. 

Why  Chlorophyll   lOOKS   green  Cells  magnified  about  200  and  molecules  about 

to  the  eye?      This,   indeed,     sixmillion  times- 
is  told  very  plainly  by  the  spectroscope,  when  it  shows  that 
chlorophyll,   in  stopping   the  useful  red  and  blue   rays  from 
the  light   of   the  sun,   allows   the  other  and   useless  rays  to 


lowing    part    for     a    time    by 

,  ~ 
itSeli. 

fW  of 

be      dismissed     VerV     brieflv 
Is  it  quite  Clear  to  the  reader 


38  The  Living  Plant 

pass  through  as  waste;  and  these  of  course  are  the  ones  which 
come  to  our  eyes.  Now  these  waste  rays  include  the  entire  green 
light,  which  gives  the  principal  color,  together  with  all  of  the  yel- 
low, which,  mixing  with  the  green,  gives  thereto  the  characteristic 
yellowish  tinge  which  chlorophyll  always  shows.  As  to  the  re- 
maining rays,  they  happen  to  form  complementary  pairs;  thus 
the  bit  of  red  and  bit  of  green-blue  form  one  pair,  while  the  orange 
and  unabsorbed  blue  form  another;  and  as  complementary  colors 
(with  lights)  always  give  white  or  gray,  these  minor  rays  thus 
neutralize  one  another  so  far  as  color  is  concerned,  and  do  not  at 
all  aft'ect  the  positive  yellow-green.  If  it  had  happened  that,  in- 
stead of  red  and  blue,  the  red  and  green  had  been  the  useful  rays, 
then  chlorophyll,  and  all  vegetation,  would  have  looked  blue;  and 
had  green  and  blue  been  the  useful  kinds,  then  all  vegetation 
would  have  looked  red.  The  greenness  of  vegetation  is  simply 
the  wastage  of  that  part  of  the  white  light  of  the  sun  which  is  not 
needed  in  photosynthesis. 

In  the  early  part  of  this  chapter  it  was  mentioned  that  many 
leaves  of  a  red  color  really  possess  chlorophyll,  which  becomes 
visible  when  the  red  is  removed  by  suitable  solvents.  This  is 
true  of  the  seaweeds,  the  red  and  brown  colors  of  which  are  due 
to  special  pigments  in  the  same  grains  with  the  chlorophyll;  and 
there  is  good  reason  for  believing  that  these  colors  bear  a  relation 
to  the  light  conditions  under  which  those  seaweeds  live,  aiding 
the  chlorophyll  to  utilize  the  sunlight  as  altered  by  its  filtration 
through  water.  The  case  in  the  more  familiar  red  plants  of  garden 
and  field,  however,  is  different.  The  colors  in  the  foliage  plants 
(Coleus,  Copper  Beeches,  Japanese  Maples)  as  well  as  in  some 
vegetables  (Beet,  Red  Cabbage),  is  a  product  of  enormous  in- 
tensification under  cultivation ;  but  in  all  cases  the  wild  ancestors 
of  these  plants  possessed  some  red  color  to  begin  with.  This  red, 
indeed,  is  fairly  common  in  wild  plants,  where  it  shows  especially 
in  veins,  petioles,  nodes,  or  the  under  sides  of  leaves,  and  in  the 
stigmas  of  many  wind-pollinated  flowers.  It  reaches,  however, 


The  Prevalence  of  Green  Color  in  Plants  39 

its  most  striking,  though  a  temporary,  development  in  the  young 
shoots  of  a  good  many  plants  (e.  g.,  Maples,  Oaks,  and  many 
herbs),  which  it  flushes  with  translucent  rose  red  as  they  push 
from  the  buds  in  the  spring,  though  later  it  fades  away  with  the 
increasing  rapidity  and  vigor  of  growth.  In  all  of  these  cases 
the  color  resides  in  a  particular  substance,  named  erythrophyll 
(or  anthocyan),  dissolved  in  the  sap  of  the  cells,  from  which.it 
can  usually  be  extracted  by  hot  water.  It  is  typically  a  beautiful 
deep  rose  red  material,  varying  much,  however,  in  tint  according 
to  the  conditions  surrounding  its  formation,  and  the  substances 
with  which  it  is  associated.  Its  identity,  therefore,  is  plain  enough, 
but  concerning  its  significance  to  the  plant  there  is  very  much 
doubt.  One  explanation  argues  thus;  erythrophyll,  as  its  color 
implies,  permits  the  red  rays  of  sunlight  to  pass  unaltered,'  but 
cuts  off,  or  at  least  weakens,  the  blue-violet  ones.  Now  it  is 
known  that  the  red  rays,  while  the  most  useful  in  photosynthesis, 
are  harmless  to  the  living  protoplasm,  but  the  blue-violet  rays, 
though  also  useful  in  photosynthesis,  are  injurious,  when  un-^ 
tempered,  to  the  living  protoplasm  and  detrimental  to  some  of 
the  physiological  processes;  therefore  (runs  the  argument),  the 
erythrophyll  probably  acts  as  a  protective  screen,  especially  in 
the  case  of  the  early  spring  vegetation,  admitting  the  beneficial 
red  rays  and  tempering  the  noxious  blue-violet  rays  until  the 
formation  of  the  chlorophyll,  which,  while  developed  for  a  differ- 
ent purpose,  incidentally  acts  as  a  protection  to  the  protoplasm,— 
a  subject  to  which,  by  the  way,  we  shall  return  for  fuller  discussion 
in  the  later  chapter  on  Protection.  A  second  explanation  is  based 
upon  the  fact  that  erythrophyll  has  been  found  to  possess  a  not- 
able power  of  transforming  light  into  heat;  it  must  therefore 
serve,  this  argument  holds,  to  warm  the  tissues  which  possess 
it,  and  thus,  during  the  bright  but  cool  days  of  the  spring,  must 
facilitate  those  processes,  such  as  nutrition,  translocation  of  food, 
and  growth,  which  are  promoted  by  warmth.  More  recently  a 
third  explanation  has  been  offered,  based  upon  the  fact  that  when- 


40  The  Living  Plant 

ever  bright  light,  a  relatively  low  temperature,  much  sugar,  and 
some  tannin  happen  to  come  together  in  a  living  cell,  then  the 
substance  erythrophyll,  of  which  the  composition-color  happens 
to  be  red,  is  formed  incidentally  as  a  purely  passive  chemical 
result.  On  this  view  the  red  color  may  be  purely  accidental, 
and  may  have  no  utility  whatever  to  the  plants  which  possess 
it,  though  the  possibility  is  not  thereby  excluded  that  the  plant 
may  bring  those  conditions  together,  adaptively,  in  a  cell  where 
it  has  need  for  the  red  color  to  appear.  The  substance  of  the  whole 
matter  in  reality  is  this, — that  we  do  not  yet  know  surely  the 
significance  of  erythrophyll  hi  the  plant;  and  herein  lies  another  of 
the  problems  which  make  science  so  ever  alluring. 

Connected  with  chlorophyll  in  a  different  way  is  one  of  the 
most  striking  and  beautiful  of  all  the  phenomena  of  nature,  the 
transition  in  the  foliage  each  season  from  the  uniform  green  of 
summer  to  the  brilliant  colors  of  autumn.  Strangely  enough,  for 
a  subject  so  important,  our  knowledge  thereof  is  still  very  im- 
perfect, and  there  is  even  a  difference  of  opinion  as  to  the  very 
significance  of  the  colors  to  the  plant.  A  basal  fact,  however, 
upon  which  there  is  agreement,  is  this, — that  the  autumn  color- 
ation results  from  changes  connected  with  the  death  and  fall  of 
the  leaf.  We  know  that  in  late  summer  our  trees  are  preparing 
for  the  annual  leaf  fall,  in  anticipation  of  which  they  are  gradually 
bringing  the  activities  of  the  leaves  to  a  close,  ceasing  to  make  new 
chlorophyll,  withdrawing  certain  precious  materials  into  the  stem, 
and  building  right  across  the  bases  of  the  leaves  those  corky  layers 
which  both  cut  them  away  from  the  stem,  and  also  heal  in  ad- 
vance the  wound  that  is  thus  to  be  made.  Now  chlorophyll,  as 
the  reader's  own  experiments  will  have  shown,  is  soon  destroyed 
by  bright  light ;  this  destruction,  indeed,  is  continually  in  progress 
throughout  the  summer  in  the  living  green  leaves,  where  the  color 
is  maintained  only  through  virtue  of  its  constant  renewal.  It 
was  formerly  believed  (and  I  mention  the  matter  because  the 
statement  persists  even  yet  in  some  writings),  that  this  chloro- 


The  Prevalence  of  Green  Color  in  Plants  41 

phyll  left  in  the  leaf  when  the  new  supply  ceases  to  form,  breaks 
down  in  the  light  to  other  substances,  which  either  themselves 
are  highly  colored,  especially  red,  or  else  unite  chemically  with 
other  materials  in  the  cells  to  form  colored  compounds,  the  autumn 
colors  being  supposed,  on  this  view,  to  be  simply  an  incidental 
product  of  chlorophyll  decay.  But  later  research  has  shown  this 
supposition  to  be  wrong,  for  chlorophyll,  in  breaking  down,  does 
not  form  colors,  but  fades  away  to  transparency  in  the  leaf  pre- 
cisely as  it  does  in  the  alcoholic  solution  which  the  reader  has 
placed  in  the  sun.  Now,  sooner  or  later  in  the  autumn  the  waning 
activity  of  the  leaf  reaches  a  point  where  no  more  chlorophyll 
is  made,  after  which  all  of  that  substance  already  present  fades 
away,  with  this  notable  result, — that  its  disappearance  renders 
visible  any  other  colors  which  may  have  been  present  in  the 
leaf,  but  masked  by  the  greater  brilliance  of  the  green;  and  this 
fact  constitutes  the  basal  step  in  the  explanation  of  autumn  color- 
ation. As  a  matter  of  fact  leaves  do  contain  other  coloring  mat- 
ters, especially  a  bright  yellow  material,  called  xanthophyll, 
occurring  in  tiny  grains  associated  with  the  chlorophyll.  It  is 
the  exposure  of  this  xanthophyll  by  the  fading  away  of  the  chloro- 
phyll which  gives  the  yellow,  most  common  of  the  autumn  colors, 
to  autumn  leaves.  If  the  reader  desires,  he  can  himself  extract 
this  xanthophyll,  and  very  easily,  in  a  beautiful  clear  yellow  solu- 
tion, by  treating  yellow  autumn  leaves  precisely  as  he  did  the 
green  leaves  for  extraction  of  chlorophyll,  but  using  much  leaf 
in  proportion  to  the  quantity  of  alcohol.  Indeed  the  reader  has 
seen  the  xanthophyll  already,  for,  as  he  will  recall,  when  he  placed 
his  solution  of  chlorophyll  in  the  sun  it  faded  away  riot  to  a  trans- 
parent whiteness  but  to  a  clear  yellow;  this  was  xanthophyll, 
which  itself  fades  away  extremely  slowly  to  whiteness.  The 
whole  situation  must  now  be  quite  clear.  Chlorophyll  and  xan- 
thophyll exist  together  in  leaves,  from  which  indeed  they  can  be 
extracted  and  separated  in  beautiful  solutions  well  known  to 
all  students  in  physiological  laboratories;  but  xanthophyll  is 


42  The  Living  Plant 

ordinarily  completely  masked  by  the  far  greater  brightness  of  the 
chlorophyll,  though  it  has  influence  enough  to  give  the  living 
leaf  its  yellow-green  rather  than  a  pure-green  color.  But  xan- 
thophyll is  vastly  more  resistent  to  the  action  of  light  than  is 
chlorophyll,  which  explains  its  persistence  in  both  leaves  and 
solutions.  The  precise  function  of  the  xanthophyll,  by  the  way, 
is  not  known,  although  it  seems  probable  that  this  is  to  be  found 
in  some  incidental  chemical  connection  with  the  chlorophyll,  in 
which  case  its  persistence  in  autumn  leaves  is  purely  incidental 
and  of  no  service  to  them. 

Second  in  abundance,  though  first  in  brilliance,  among  autumn 
colors  is  red,  which  has  a  very  different  origin.  It  is  due  to  the 
presence  of  that  same  erythrophyll  which  we  have  already  con- 
sidered in  connection  with  foliage  plants  and  the  spring  coloration. 
This  erythrophyll,  also,  the  reader  can  extract  for  study  in  a  beau- 
tiful clear  rose-red  solution  by  aid  of  the  method  he  used  for  the 
chlorophyll,  excepting  that  water  must  be  used  instead  of  alcohol, 
and  the  material  should  be  abundant  and  consist  of  the  very 
brightest  red  leaves  he  can  find.  Unlike  the  xanthophyll  the 
erythrophyll  is  not  present  in  the  leaves  before  the  chlorophyll 
fades  away,  at  least  not  in  appreciable  amount;  but  it  forms  as 
the  disappearance  of  the  chlorophyll  admits  the  light  to  the  in- 
terior of  the  leaf  cells.  That  the  presence  of  bright  light  is  es- 
sential to  its  formation  is  easily  proven  by  experiment,  and  by 
the  readily  observable  fact  that  in  cases  where  one  red  autumn 
leaf  overlaps  another  closely  enough  to  shield  it  largely  from  light, 
the  darkened  portion  is  yellow  not  red;  and  this  same  fact  further 
proves  that  red  autumn  leaves  are  actually  yellow  underneath  the 
red.  The  brilliancy  of  the  red,  indeed,  is  proportional  in  general 
to  the  brightness  of  the  light.  But  light  alone  is  not  sufficient 
to  produce  a  formation  of  erythrophyll  without  the  presence  of 
the  chemical  substances  requisite  to  its  formation,  which  include 
certainly  sugar  and  probably  tannin;  and  it  is  only  those  leaves 
which  happen  to  contain  a  sufficiency  of  these  materials  that  can 


.    •-• 


\ 


PLATE     II 


PLATES  II  and  III 

Typical  Autumn  colors  in  common 
New  England  plants 


PLATE     III 


./,v 

*1 


The  Prevalence  of  Green  Color  in  Plants  43 

turn  red  at  all,  the  others  being  restricted  to  yellow.  The  Maples 
and  the  Oaks  are  trees  well-known  for  their  richness  in  sugar  or 
tannin,  which  helps  to  explain  why  those  particular  trees  are 
more  brilliantly  red  than  most  others.  It  happens,  furthermore, 
that  erythrophyll  formation,  contrary  to  the  usual  rule  with 
chemical  processes,  is  promoted  by  lower  temperature;  and  this 
explains  why  it  is  that  a  cool  season  promotes  the  brilliance  of 
color,  which  indeed  reaches  its  highest  perfection  in  seasons  or 
places  where  the  skies  are  very  bright  and  the  frosts  come 
early. 

Thus  much  for  the  facts  as  to  the  yellow  and  red  autumn  color- 
ation. We  have  now  to  take  notice  that  two  conflicting  views 
exist  as  to  its  significance  to  plants.  Many  botanists  believe  that 
since  erythrophyll  seems  to  have  definite  functions  in  spring 
vegetation  (as  we  have  seen  a  few  pages  earlier),  it  has  also  an 
identical  function  in  the  leaves  of  the  autumn,  acting  usefully 
as  a  selective  light  screen.  The  argument  runs  thus: — chloro- 
phyll fades  away  in  the  leaf  before  the  protoplasm  has  wholly 
ceased  its  activity:  full  exposure  to  bright  sunlight,  especially 
the  untempered  blue-violet  rays,  would  injure  this  protoplasm, 
and  act  unfavorably  on  the  translocation  of  the  valuable  materials 
from  the  leaf  into  the  stem :  an  erythrophyll  screen  must  temper 
the  blue-violet  rays  while  permitting  the  passage  of  the  red  rays 
which  are  not  simply  harmless  but,  being  warm  rays,  would  actually 
aid  the  final  vital  processes  of  the  leaf  during  the  cooling  days  of 
autumn.  And  those  who  hold  this  view  assume  that  xanthophyll 
must  have  something  of  the  same  action,  though  inferior  in  degree 
to  erythrophyll.  On  this  view  autumn  colors  are  believed  to  be 
useful  if  not  indispensable  to  the  plants  which  possess  them,  and 
inferentially,  have  been  developed  adaptively  to  such  use. 

Sharply  contrasting,  however,  with  this  utility  explanation  of 
autumn  coloration  is  the  view  that  it  is  merely  incidental.  While 
the  utility  theory  has  certainly  some  facts  in  its  favor,  the  most 
of  the  evidence  seems  to  me  heavily  against  it.  Thus  the  utility 


44  The  Living  Plant 

theory,  that  of  the  protective  and  heating  screen,  requires  in 
autumn  leaves  certain  features  which  the  spring  coloration  does 
in  fact  to  some  extent  exhibit, — viz.,  a  prevalence  of  red  rather 
than  yellow,  a  fairly  uniform  coloration  over  all  the  parts  to  be 
protected  or  warmed,  an  especially  deep  coloration  in  the  conduct- 
ing parts,  and  a  fairly  constant  development  of  the  color  year  after 
year  without  much  regard  to  the  details  of  the  weather.  As  a 
matter  of  fact  the  phenomena  of  autumn  coloration  are  differ- 
ent at  almost  every  point — red  is  less  common  than  yellow;  the 
colors  are  very  uneven  in  distribution,  forming  spots,  blotches, 
and  streaks;  the  color  shows  no  particular  tendency  to  cover  the 
conducting  veins:  and  its  intensity  varies  greatly  in  different 
years,  even  almost  to  suppression  of  red  in  certain  kinds  of  leaves 
in  some  seasons.  The  utility  theory  of  autumn  coloration  re- 
ceives, therefore,  no  support  from  comparison  with  spring  color- 
ation, even  granting,  as  is  not  at  all  certain,  that  the  latter  is 
useful.  The  facts,  therefore,  taken  all  together  seem  to  favor 
the  incidental  theory,  which  may  thus  be  expressed ; — that  autumn 
coloration,  for  the  most  part  at  least,  is  a  purely  incidental  result 
of  the  chemical  and  physical  conditions  which  happen  to  prevail 
in  ripening  leaves  and  around  them,  and  has  in  it  no  more  element 
of  utility  than  has  the  red  of  a  sunset  or  the  blue  of  the  firmament. 
The  yellow  and  red  in  the  autumn  coloration  are  so  much  more 
common  and  striking  than  any  other  colors  that  they  naturally 
attract  the  most  of  our  attention.  Yet  other  colors  occur,  as 
everybody  knows  well,  and  as  appears  very  clearly  on  the  accom- 
panying plates  (Plates  II,  III),  which  represent  a  selection  from 
New  England  autumn  vegetation,  photographed  in  the  natural 
colors.  In  fact,  however,  the  great  variegation  thus  displayed 
results  from  permutations  and  combinations  of  a  very  few  colors. 
In  addition  to  the  red  and  yellow,  there  is  only  one  other  pigment 
at  all  common  in  autumn  leaves,  and  that  is  an  occasional  brown, 
the  mode  of  formation  of  which  is  uncertain.  Most  of  the  brown 
color  in  such  leaves,  however,  belongs  to  the  cell-walls,  which  are 


The  Prevalence  of  Green  Color  in  Plants  45 

white-transparent  when  alive,  but  turn  brown  on  their  death  and 
decay.  In  fact  the  conditions  prevailing  in  the  ripening  and  dying 
leaf  are  most  complex,  for  not  only  are  different  chemical  sub- 
stances and  physical  forces  interacting  in  large  number,  but  their 
interrelations  are  constantly  changing  as  the  death  of  the  proto- 
plasm weakens  its  regulatory  control  upon  them.  This  combina- 
tion of  complexity  and  changeability  produces  a  state  of  unstable 
equilibrium,  which  permits  even  very  minor  external  influences 
to  exert  relatively  great  effects, — and  thus  is  explained  the  differ- 
ences in  the  coloration  of  the  same  plants  in  different  seasons  or 
different  places.  In  general,  however,  the  effects  of  the  weather 
upon  the  intensity  of  coloration  are  clear.  Thus  a  bright  autumn 
(and,  equally,  a  sunny  climate)  intensifies  the  coloration,  at  least 
for  the  red,  while  dull  weather  is  accompanied  by  dull  coloration. 
Early  frost  helps  somewhat  to  intensify  color,  partly  by  hastening 
the  death  of  the  leaf,  and  partly  by  aiding  the  chemical  formation 
of  the  erythrophyll ;  though  frost  is  not,  as  many  suppose,  a  cause 
of  the  coloration  itself.  Furthermore,  the  coloration  can  be 
brought  on  much  earlier  in  the  season  than  usual  by  any  injury, 
— a  break  in  the  bark,  a  split  in  the  trunk,  some  damage  to  the 
roots, — which  weakens  the  vitality  of  the  tree  and  hence  pro- 
motes the  waning  of  life  in  the  leaves;  and  this  is  the  explana- 
tion of  the  occasional  reddening  of  a  single  branch,  or  even 
whole  tree,  which  one  finds  turning  sometime  ahead  of  its 
neighbors. 

The  reader  will  feel,  I  am  sure,  that  this  is  an  unsatisfying  answer 
to  his  natural  wish  for  a  definite  knowledge  of  the  causes  of 
autumn  coloration,  but  it  is  all  that  the  present  state  of  our 
knowledge  permits.  The  subject  has  been  studied  heretofore 
by  botanists  from  their  side,  and  by  chemists  from  theirs;  but 
its  problems  will  not  be  solved  until  some  competent  investiga- 
tor takes  autumn  coloration  as  his  unit,  and  attacks  it  by  any  and 
all  methods, — chemical,  physical,  physiological,  observational, 
experimental,  or  any  others  essential  for  attaining  his  ends. 


46  The  Living  Plant 

Some  day  this  will  be  done,  and  then  we  shall  know  the  meaning 
of  autumn  coloration  just  as  surely  as  we  now  know  the  causes 
of  the  colors  of  chlorophyll,  of  fruits,  and  of  flowers.  Meantime, 
it  is  not  the  least  of  the  pleasures  of  science  that  everywhere  about 
us  lie  problems  of  moment,  whose  progress  towards  solution  we 
may  constantly  watch,  and  the  triumph  of  whose  conquest  we 
may  perhaps  even  share. 


CHAPTER  III 

THE  PROFOUND  EFFECT  ON  THE  STRUCTURE  OF  PLANTS 
PRODUCED  BY  THE  NEED  FOR  EXPOSURE  TO  LIGHT 

Morphology  and  Ecology  of  Leaves  and  Stems 


N  the  foregoing  chapter  we  have  considered  photo- 
synthesis solely  as  a  physiological  process  operating 
within  the  body  of  the  plant,  and  have  taken  no  thought 
for  any  relations  it  may  have  with  the  world  outside. 
Yet  the  internal  process  is  dependent  on  the  external  world  in 
this  very  fundamental  particular,  that  the  supply  of  the  indis- 
pensable light,  carbon  dioxide  and  water  has  to  come  from  out- 
side. Furthermore,  and  this  is  a  point  of  importance,  the  en- 
vironment rarely  offers  these  essentials  in  precisely  the  right 
quantities,  but  sometimes  too  abundantly,  oftener  too  sparsely, 
and  sometimes  in  ways  involving  grave  dangers.  Their  photo- 
synthetic  needs  plants  cannot  help,  and  their  environmental 
conditions  they  cannot  change,  but  there  is  one  thing  that  is  al- 
terable, and  that  is  their  own  structure,  with  its  large  poten- 
tialities of  adaptive  development.  Accordingly,  in  the  course  of 
long  ages  of  slow  evolution,  plants  have  become  so  molded  in 
form  and  in  structure  as  to  bring  the  photosynthetic  process 
into  advantageous  or  adaptive  relation  with  the  conditions  of 
supply  of  the  photosynthetic  essentials  outside,  and  in  such  man- 
ner, moreover,  as  to  permit  of  particular  adjustment  to  special 
peculiarities  of  the  surroundings.  Plants  are  like  housekeepers 
who  possess  certain  needs,  and  a  desire  for  having  the  best,  but 
who  have  no  control  over  the  purse-strings ;  under  the  circumstances 
there  is  nothing  for  them  to  do  but  adjust  the  scale  and  style  of 

47 


48  The  Living  Plant 

the  establishment  to  the  exigencies  of  a  fixed  income.  This  is 
the  real  meaning  of  the  photosynthetic  adaptations,  which  it  is 
now  our  business  to  consider.  Each  one  of  the  physiological 
processes  of  plants  produces,  of  course,  in  like  manner  its 
effect  upon  their  structure;  but  the  one  process  of  photosynthesis 
far  surpasses  all  others,  indeed  all  others  put  together,  in 
the  profundity  of  its  influence  in  making  plants  what  they 
actually  are.  The  evidence  thereof  will  appear  in  the  following 
pages. 

The  photosynthetic  essentials  for  which  plants  are  dependent 
upon  the  environment  are  in  reality  four,  because,  in  addition  to 
light,  carbon  dioxide,  and  water,  plants  need  also,  for  reasons  that 
will  later  appear,  certain  minerals,  which  are,  however,  for  the 
most  part  very  widely  distributed  in  soils.  Now  in  showing  the 
way  in  which  these  four  are  supplied  by  the  environment  to  plants, 
I  must  recall  to  the  reader  some  very  familiar  and  commonplace 
facts.  But  I  remind  him  that  there  is  nothing  in  the  world  so 
difficult  to  see  in  its  real  significance  as  the  commonplace;  more- 
over let  him  remember  the  truth  expressed  by  a  brilliant  writer 
in  the  saying  that  little  minds  are  interested  in  the  extraordinary, 
but  great  minds  in  the  commonplace. 

The  crucial  facts  about  the  mode  of  supply  of  the  four  photo- 
synthetic  essentials  are  these. 

First.  They  all  exist  widely  even  if  not  abundantly  distributed  in 
nature,  and  moreover  are  incessantly  in  movement  or  circulation, — 
the  light  with  the  swing  of  the  sun  through  the  heavens,  the  car- 
bon dioxide  with  every  breeze  that  stirs  the  still  air,  the  water 
in  the  form  of  the  mists  and  the  rain,  and  the  minerals  in  solution 
in  the  water  which  soaks  and  drains  through  the  soil.  Therefore 
plants  have  no  need  to  go  in  search  of  these  essentials,  as  animals 
must  for  their  food,  but  are  able  to  stay  fixed  in  one  place  and 
allow  the  essentials  to  be  brought  them  by  the  general  circula- 
tion of  nature.  This  method  renders  needless  any  self-motive 
power,  with  the  accompanying  muscular  system  and  jointed  skele- 


The  Profound  Effect  on  the  Structure  of  Plants      49 

ton  such  as  animals  must  have,  and  permits  a  simply  continuous 
structure.  This  is  why  plants  are  sedentary  beings,  rooted  for  life 
in  one  spot, 

Second.  The  four  essentials  circulate  in  no  definite  paths  or 
directions,  but  come  to  the  plant  from  every  point  of  the  compass. 
This  is  true  even  of  sunlight,  despite  the  regular  path  of  the  sun 
through  the  heavens,  for  so  uniform  is  the  diffusion  of  the  light 
through  the  sky  that  plants  really  receive  it  from  every  direction. 
And  as  to  the  wind,  does  it  not  blow  where  it  listeth,  and  the 
waters,,  do  they  not  cover  the  earth?  Therefore  plants  have  no 
need  to  face  their  parts  in  any  particular  direction,  as  animals 
must  do  hi  connection  with  their  movements  in  search  of  their 
food,  but  face  evenly  outward  in  every  direction,  thus  requiring  a 
symmetrical  distribution  of  their  parts  around  a  central  vertical 
axis.  This  is  why  plants  are  radially  built,  presenting  the  same 
face  to  all  points  of  the  compass. 

Third.  The  four  essentials  are  not  evenly  commingled,  but  seg- 
regated into  two  strata, — the  light  and  carbon  dioxide  in  the  at- 
mosphere above,  and  the  water  and  minerals  in  the  soil  under- 
neath. Therefore  plants  must  needs  have  two  parts  to  their 
structure  adapted  to  life  in  these  two  very  different  situations. 
This  is  why  plants  exhibit  their  primary  division  of  structure  into 
the  green  shoot  (leaf  and  stem),  and  colorless  root. 

Fourth.  The  four  essentials  exist  rarely  in  abundance  and  then 
never  for  much  of  the  time,  and  most  commonly  are  sparser  than 
plants  can  make  use  of.  Frequently  the  light,  always  the  carbon 
dioxide,  often  the  water,  and  sometimes  the  minerals  are  accessi- 
ble only  in  dilution.  Therefore  the  plant  must  needs  reach  out 
extensively  to  come  into  contact  with  a  sufficiency, — a  condition  in 
great  contrast  to  that  prevailing  in  animals  with  their  concen- 
trated food  and  consequent  compactness  of  body.  This  is  why 
plants  are  branched  so  profusely  and  slenderly. 

Fifth.  One  of  the  four  essentials, — viz.,  light,  is  of  such  nature 
that  it  cannot  be  transmitted  far  into  the  plant,  and  therefore  must  be 


50  The  Living  Plant 

used  at  the  surface.  Hence  plants  have  had  to  distribute  the  green 
tissues  of  the  shoot  in  a  manner  ensuring  the  exposure  of  a  great 
spread  of  surface  to  light,  and  this  involves  a  flattening  of  most 
of  the  tissues  of  the  shoot  to  the  thinnest  practicable  structures. 
This  is  why  leaves  exist,  and  why  the  green  plant  consists  of  them  so 
largely. 

Sixth.  One  of  the  essentials,  the  sunlight,,  falls  upon  plants  from 
every  direction  in  the  aerial  hemisphere.  Not  only  does  it  come  from 
a  source  which  forever  is  changing  its  position  in  the  skies,  but, 
furthermore,  this  light  is  so  strongly  diffused  through  the  atmos- 
phere that  it  falls  upon  plants  from  every  direction  in  an  in- 
tensity which  for  most  of  the  time  is  as  great  as  leaves  can  make 
use  of;  for  it  is  a  physiological  fact  that  plants  cannot  use  all  the 
energy  contained  in  full  sunlight,  and  strong  diffused  light  is 
enough  for  their  needs.  Hence  it  comes  to  pass  that  plants 
receive  light  in  amount  and  direction  sufficient  to  illuminate  a 
great  many  leaves  if  only  these  are  carried  to  various  heights  and 
spaced  well  apart,  in  a  general  distribution  answering  to  that 
of  the  incident  light.  This  necessitates  the  specialization  of  a 
part  of  the  shoot  for  carrying  the  leaves  upwards  and  outwards. 
This  is  the  reason  why  stems  exist  and  branch  in  such  manner  as 
typically  to  carry  the  leaves  to  a  hemisphere  of  foliage. 

Thus  it  is  evident  that  the  most  distinctive  features  of  struc- 
ture and  form  displayed  by  plants  of  the  highest  development, 
the  features  indeed  which  are  most  closely  associated  with  our 
very  idea  of  plants, — the  sedentary  habit,  the  radial  symmetry, 
the  diffuse-slender  branching,  the  primary  division  into  shoot 
and  root,  and  of  the  shoot  into  flat  leaves  and  supporting  stems, — 
all  exist  as  adaptations  which  adjust  the  photosynthetic  process 
to  the  conditions  under  which  the  photosynthetic  essentials  are 
supplied  by  the  external  world.  It  is  therefore  a  fact  that  the 
photosynthetic  process  determines  the  ground  form  and  primary 
structure  of  plants  just  as  truly  as  it  determines  their  ground 
color. 


The  Profound  Effect  on  the  Structure  of  Plants      51 


FIG.  7. — The  form,  as  seen  in  vertical 
section,  which  a  plant  would  display 
(theoretically)  if  free  to  adapt  itself  to 
photosynthesis  alone.  Further  particu- 
lars in  the  text. 


It  is  worth  while  to  try  to  express  the  sum  of  these  features 
in  diagrammatic  form,  and  my  suggestion  thereof  is  contained 
in  figure  7.  The  purely  photosynthetic  plant  would  exhibit 
a  system  of  equal  rigid  branches 
springing  as  radii  from  a  central 
trunk,  and  forking  regularly 
outward  to  a  vast  number  of 
young  twigs  which  would  turn 
up  near  the  tips  to  spread  the 
leaves  horizontally  in  a  hollow 
hemisphere  of  foliage.  This 
theoretical  form,  of  course,  is 
modified  in  practice  by  other 
considerations,  especially  the 
exigencies  of  mechanical  sup- 
port, as  we  shall  later  consider; 

but  nevertheless  it  comes  appreciably  close  to  realization  in  the 
most  typical  of  the  great  trees,  when  these  are  free  to  develop 
without  interference,  as  was  the  case  with  the  Oak  of  the  ac- 
companying picture  (figure  8). 

We  turn  now  to  a  particular  study  of  those  two  most  distinctive 
plant  structures,  the  leaf  and  the  stem.  A  first  view  over  leaves 
in  general  gives  only  the  impression  of  bewildering  multiformity; 
but  continued  observation  gradually  sorts  out  the  important 
from  the  trivial,  and  builds  one  of  those  visualized  composites  of 
which  I  have  spoken  in  the  first  chapter.  As  the  reader  should 
review  and  confirm  for  himself  by  inspection  of  a  number  of 
kinds  brought  together  for  the  purpose,  the  principal  part  of  an 
ordinary  leaf  is  the  spreading  thin  blade,  which  exhibits  two  con- 
stituents,— first,  the  soft,  seemingly-homogeneous,  chlorophyllous 
tissue,  denser  in  green  on  the  uppermost  surface,  and  seat  of  the 
food-making  process,  and  second,  the  slender  white  veins,  spring- 
ing out  from  the  leaf-stalk  and  variously  branching  and  inter- 
lacing while  ever  attenuizing  towards  the  margin  and  tip  of  the 


52  The  Living  Plant 

blade.-  The  tiniest  veins  are  embedded  within  the  green  tissue, 
where  they  end  in  polygonal  areas,  as  one  can  see  with  a  lens  in 
some  leaves  by  holding  them  up  to  the  light  (for  example  in  Rose, 
Cabbage,  and  Wild  Ginger),  and  as  shown  in  the  accompanying 
cut  (figure  9) ;  but  the  larger  veins  stand  out  from  the  surface, 
though  always  from  the  undermost  side  where  they  are  out  of 
the  way  of  the  light.  The  veins  have  a  double  function, — the 
conduction  of  water  from  the  stem  to  the  green  tissue,  and  the 


FIG.    8. — An  oak  tree,  showing  an  approximation  to  the  theoretical  form  of  figure  7. 
(Copied  from  Blanchan's  American  Garden.) 


conduction  of  the  photosynthetic  sugar  back  to  the  stem;  and 
they  have  also  a  secondary  use  in  helping  a  little  to  support  the 
soft  tissue,  though  the  rigid  but  elastic  stiffness  of  the  healthy 
green  leaf  is  due  for  the  most  part  to  osmotic  turgescence,  of 
which  I  shall  speak  in  the  suitable  place.  In  addition  to  the  blade, 
most  leaves  possess  a  leaf-stalk,  or  petiole,  stem-like  in  appear- 
ance and  function  and  varied  in  length,  which  carries  the  blade 
out  into  the  light  and  aids  to  adjust  it  therein,  as  we  shall  later 


The  Profound  Effect  on  the  Structure  of  Plants      53 

consider  more  fully  under  light-adjustment,  or  phototropism. 
Finally,  some  leaves  exhibit,  just  where  the  petiole  joins  the  stem, 
a  pair  of  little  leaf-like  bodies  called  stipules,  whose  most  remark- 
able feature  is  the  diversity  of  their  somewhat  insignificant 
functions  and  forms.  All  of  the  parts  of  a  typical  leaf, — blade, 
petiole  and  stipules, — are  well  shown  and  in  typical  form,  in  the 
accompanying  picture  (figure  10). 


FIG.  9. — A  fragment  of  the  vein  system  of  a  leaf,  highly  magnified,  showing  the  typical 
mode  of  ultimate  branching  and  ending  of  the  veinlets.  (From  Sachs'  Lectures, 
reduced.) 

FIG.  10.— A  typical  leaf,— the  Quince.     (From  Gray's  Text-books). 


The  most  striking  of  the  features  of  leaves  is  perhaps  the  re- 
markable variety  of  their  shapes,  which  seem  in  their  multiform- 
ity to  defy  explanation  or  classification.  Yet  in  reality  the  matter 
is  simple,  for  there  exist  only  three  primary  forms  of  which  all 
the  others  are  modifications  and  combinations,  as  the  following 
analysis  will  show. 

First,  the  ideal  condition  for  the  best  working  of  a  leaf  is  ob- 
viously that  in  which  it  can  have  full  exposure  to  all  the  light  that 


FIG.  11. — Leaves  selected  to  illustrate  the  typical  shapes;  a  photograph  of  living  specimens, 
one-third  the  natural  size. 


54 


The  Profound  Effect  on  the  Structure  of  Plants      55 

there  is,  without  any  shading  by  its  neighbors.  This  ideal  ex- 
posure allows  the  development  of  the  ideal  type  of  construction, 
i.  e.,  the  shape  that  encompasses  the  most  green  tissue  within 
the  least  outline,  and  a  venation  ensuring  the  shortest  paths  for 
conduction  of  water  and  the  photosynthate.  Such  a  leaf  must  be 
round,  with  its  veins  radiating  from  a  central  petiole.  It  is  well- 
nigh  realized  in  the  leaf  of  the  Common  Garden  Nasturtium 
(figure  11,  c),  a  low-stemmed  plant  whose  long  petioles  permit 
a  full  exposure  of  each  leaf  to  light  (figure  12) ;  and  it  is  shown  con- 
ventionalized in  figure  13,  a.  Furthermore,  this  association  of 
round-radiate  (or,  in  the  current  terminology,  round-palmate), 
shape  with  full  exposure  to  light  is  actually  found  in  most  plants 
which  grow  in  such  manner  that  their  leaves  do  not  shade  one 
another,  as  for  example  in  the  floating  leaves  of  Water  Lilies 
(figure  11,  a),  Ground  Ivy  (figure  11,  6),  Wild  Ginger,  and  others 
which  trail  or  creep  on  the  ground,  and  in  low-growing  long- 
petioled  herbs  like  Geranium,  Cyclamen,  and  Pelargonium,  and 
partially  in  Ivies.  Most  of  these  leaves  show  a  slit  from  the 
petiole  to  margin,  but  that  does  not  alter  the  principle  of  the 
central-standing  petiole,  for  the  slit  is  merely  a  relic  of  the  evolu- 
tion of  these  leaves  from  kinds  in  which  the  petiole  stood  on  the 
margin;  indeed  all  intermediate  gradations  exist  in  heart-shaped, 
arrow-shaped,  and  "auriculate"  leaves,  where  a  part  of  the  blade 
bulges  backward  on  each  side  of  the  petiole. 

Second,  the  opposite  extreme  of  habit  is  found  where  leaves 
are  compelled  to  grow  crowded  together,  as  they  are  in  most  plants 
living  in  especially  dry  or  light  places.  In  this  case  the  best  shape 
and  arrangement  would  be  necessarily  the  exact  opposite  of  those 
found  in  the  round  type,  that  is,  the  leaves  would  be  slender  or 
linear,  without  distinction  of  petiole  and  blade,  and  with  the  veins, 
running  parallel;  while  they  would  take  such  positions  as  would 
admit  the  light  most  deeply  and  evenly  among  them, — viz.,  they 
would  point  at  the  light  and  therefore  stand  parallel  or  radiating 
with  respect  to  one  another.  Such  a  position  for  the  leaves  is  in 


56  The  Living  Plant 

fact  not  at  all  bad  for  illumination,  since  diffused  light  can  pen- 
etrate rather  deeply  among  them,  while  the  sun,  in  its  daily  swing 
through  the  heavens,  slants  its  beams  at  times  to  the  innermost 
parts  of  them  all.  The  typical  linear  shape  is  actually  realized 
in  a  great  many  leaves,  of  which  our  figure  1 1  shows  a  few  (/,  g,  h) ; 
and  it  is  shown  conventionalized  in  figure  13,  6.  The  association 
of  linear  shape  with  a  crowding  of  leaves  into  dense-radiating 
heads  is  found  typically  developed  in  a  good  many  plants,  such  as 


Fio.  12. — The  three  types  of  plant  form  with  which  are  associated  the  three  fundamental 
types  of  leaf  shape.  On  the  left  is  the  trailing  Garden  Nasturtium,  in  the  middle, 
the  half-desert  Cordyline,  on  the  right  the  typical  woods-plant  Ficus  religiosity. 


Spanish  Bayonets,  and  the  remarkable  Tree  Yucca  of  the  deserts, 
in  Century  Plants,  the  ornamental  Cordylines  (figure  12),  and 
some  of  the  Bunch  Grasses.  The  association  of  the  linear  form 
with  parallel-standing  leaves  is  realized  in  the  Flags  and  Cat- 
tails of  stream  margins,  and  especially  in  the  Grasses  of  the 
meadows,  which  thus  crowd  a  vast  number  of  leaves  into  a  lim- 
ited area.  And  another  phase  of  the  very  same  thing  is  presented 
by  some  of  our  evergreen  trees,  with  their  linear  or  needle-shaped 


The  Profound  Effect  on  the  Structure  of  Plants        57 


Third,  the  conditions  to 
which  are  adjusted  the  round 
and  the  linear  shapes  of 
leaves  are  uncommon  in 
comparison  with  that  in 
which  numerous  leaves  are 
spaced  at  different  heights 
along  ascending  stems, — 
for  this  latter  is  the  prevail- 
ing mode  in  vegetation, 
(figure  12,  right).  Since  this 

leaves.      These  symmetri-     II  I  111    condition    is    intermediate 
cal  cone-shaped  trees  may  I   between  the  other  two,  we 

be  viewed,  indeed,  as  a  se-  I   anticipate  an   intermediate 

ries  of  superposed  meadows,  I  shape  of  leaf,  which  would 

spaced  well  apart  in  stories  1 1  1 1  therefore  be  elliptical  in  out- 

so  arranged  that  each  is 
smaller  than  the  one  next 
beneath  it,  thus  avoiding 
injurious  shading  thereof, 
while  the  leaves  point  out- 
ward as  well  as  upward  to- 
wards the  strongest  light. 
This  condition  is  repre- 
sented diagrammatically  in 
figure  14,  and  it  comes  very 
close  to  actual  realization 
in  some  of  our  Spruces  and 
Firs  when  these  are  free  to 
develop  as  they  will  (figure 
15).  *  This  is  the  principal 
factor,  I  believe,  in  the  ex- 
planation of  the  conical 
form  of  the  evergreens. 

FIG.    13.— Conventionalizations  of  the  three  fundamental  types  of  leaf  form. 


58  The  Living  Plant 

line  with  the  petiole  at  one  end  and  the  veins  branching  off  pin- 
nately  from  an  axial  mid-rib.  This  shape  and  venation  are 
actually  realized  in  the  leaves  of  some  trees,  very  typically  in 
Chestnut  (figure  11,  d),  Elm,  Rubber-plant,  and  Banana.  Much 

oftener,  however,  this  outline  is 
modified  by  a  condensation  of 
the  green  tissue  towards  the 
base  of  the  leaf,  which  ensures 
a  shorter  path  of  conduction 
for  water  and  the  photosyn- 
thate,  while  lessening  simul- 
taneously the  weight  and  lev- 
&2&y  erage  on  the  petiole.  Such 
^^^g^  leaves  are  necessarily  of  ovate 
outline,  and  these  ovate-pinnate 
leaves  are  very  common  in  na- 
ture. The  shape  is  well  typi- 

FIG.  14. — The  theoretical  form,  seen  in  ver- 
tical section,  of  an  evergreen  tree.     Further    fied    in    the     Catalpa,     for     CX- 
particulars  in  the  text.  ,          ,„  •,     . 

ample,   (figure    11,  e),  and  is 

represented  in  conventionalized  form  in  our  figure  13,  c.  In 
some  plants  the  condensation  goes  so  far  as  to  make  the  leaf  al- 
most round,  as  for  example  in  the  Red-bud  (figure  11,  i),  when 
the  venation  makes  some  approach  to  the  palmate  type  and  the 
petiole  is  apt  to  be  notably  long.  Such  leaves  often  show  a  bulge 
of  the  tissue  downward  each  side  of  the  petiole,  thus  displaying 
a  transition  to  the  typical  round  shape  with  which  we  began. 

It  is  thus  evident  that  three  fundamentally-distinct  condi- 
tions of  leaf  exposure  exist,  with  three  corresponding  types 
of  leaf  shape, — the  round-radiate,  the  linear-parallel,  and  the 
ovate-pinnate.  But  innumerable  intermediate  conditions  of  leaf- 
habit  exist,  and  therefore  innumerable  intermediate  leaf  shapes 
occur.  These  shapes  have  a  large  practical  importance  in  the 
classification  and  description  of  plants,  and  accordingly  have  been 
named  for  this  purpose  with  very  great  accuracy;  and  it  is  inter- 


The  Profound  Effect  on  the  Structure  of  Plants       59 

esting  to  note  that  while  some  of  the  shapes  have  been  named  for 
their  resemblance  to  familiar  mathematical  forms  or  common 
objects  (e.  g.,  ovate,  lanceolate),  the  majority  have  to  be  desig- 
nated by  combinations  of  these  terms  (as  ovate-lanceolate,  etc.). 

For  completion  of  our  subject 
of  leaf  shape,  one  matter  of  im- 
portance remains,  and  that  con- 
cerns the  curious  emarginations, 
lobings,  and  compoundings  which 
so  many  of  the  kinds  exhibit. 
The  margin  of  a  leaf  is  typically 
smooth  or  entire,  and  many  leaves 
actually  exhibit  this  character; 
but  others  again  are  more  or 
less  waved,  toothed,  or  incised, 
through  the  sagging,  as  it  were, 
of  the  green  tissue  between  the 
ends  of  the  veins,  or,  occasionally, 
its  swelling  out  beyond  them. 
When  this  lobing  becomes  deep, 
it  influences  greatly  the  form  of 
the  leaf,  especially  as  it  follows 
the  type  of  the  veining.  Thus, 
a  deep  lobing  between  palmate 
veins  results  in  a  shape  like  that 
of  the  Ivies,  and  the  Maples 
(figure  11,  j],  while  if  it  goes  clear  down  to  the  leaf-stalk  (in  which 
case  the  separated  segments  usually  develop  little  stalks  of  their 
own),  it  results  in  a  leaf  that  is  palmately  compounded,  like 
the  Woodbine  (figure  11,  k).  A  similar  deep  lobing  in  pinnately- 
veined  leaves  leads  through  forms  like  those  of  the  Oaks  to 
pinnately-compound  leaves,  like  those  of  the  Locust  (figure 
11,  /)  and  many  Ferns,  which  latter,  indeed,  are  often  again  lobed 
and  compounded,  and  re-compounded  again.  In  a  general  way, 


FIG.  15. — Engelmann's  Spruce,  showing 
an  approximation  to  the  theoretical 
form  of  figure  14.  (Copied  from  Kirke- 
gaard's  Practical  Handbook  of  Trees, 
etc.) 


60  The  Living  Plant 

as  will  later  appear,  there  is  a  probable  adaptational  advantage 
in  the  compounding  of  leaves,  since  it  aids  them  to  resist  the 
tearing  action  of  strong  winds,  and  there  is  a  possible  adaptive 
explanation  of  the  deep  lobing  of  leaves  like  Ivies  and  Maples 
in  the  opportunity  thus  afforded  for  an  interlocking  of  the  leaves 
and  consequent  utilization  of  every  ray  of  the  incident  light. 
But  nobody,  so  far  as  I  can  find,  has  yet  been  able  to  give  a  reason- 
able explanation  of  the  significance  of  the  emarginations  of  leaves, 
for  the  suggestion  that  the  points  thus  resulting  serve  to  collect 
atmospheric  electricity  for  some  use  by  the  leaf  can  hardly  be 
seriously  entertained.  Emargination,  lobing  and  compounding 
are  evidently  three  degrees  of  the  same  thing,  but  it  is  by  no  means 
necessary  to  believe  that  because  compounding  is  adaptively 
useful,  therefore  emargination  must  be  useful  likewise.  On  the 
contrary,  it  is  not  only  possible  that  the  emargination  of  leaves 
originates  non-adaptively  in  some  manner  purely  incidental 
or  accidental,  and  is  later  intensified  adaptively  to  lobing  and 
compounding,  but  the  method  embodied  in  this  supposition  affords 
the  most  reasonable  explanation  we  yet  possess  of  the  origin  of 
adaptations. 

While  adaptation  to  the  mode  of  exposure  to  light  is  the  chief  fac- 
tor in  determining  the  shape  of  the  leaf,  other  adaptations  and  influ- 
ences, very  different  in  different  cases,  exert  also  their  effects, 
making  the  shape  of  any  given  leaf  a  resultant  of  the  cooperation  of 
many  influences.  This  fact  the  reader  must  remember  when  he 
tries  to  apply  the  principles  of  the  preceding  pages  to  the  ex- 
planation of  leaf  shapes  he  may  find  in  his  walks  abroad  in  the 
country.  At  first  he  will  find  so  many  exceptions  and  contra- 
dictions that  he  may  incline  to  dismiss  my  explanations  as  ground- 
less; but  if  he  will  continue  his  observations  with  patience,  he 
will  gradually  find  the  exceptions  disappearing  and  the  essentials 
standing  out  in  those  composite  conceptions  of  which  I  have 
spoken  in  the  first  chapter;  and  then,  I  believe,  he  will  agree 
with  the  conclusions  here  expressed. 


The  Profound  Effect  on  the  Structure  of  Plants      61 

From  the  leaf  we  turn  to  the  associated  and  well-nigh  equally 
distinctive  part,  the  stem,  of  which,  however,  the  structure  is 
comparatively  simple  and  uniform.  Since  its  principal  function 
consists  in  raising  and  spreading  a  great  many  leaves  to  the  light, 
it  must  of  course  be  adapted  to  provide  a  firm  mechanical  support 
in  conjunction  with  much  branching;  and  in  fact  it  consists  of 
a  cylindrical-tapering,  rigid-continuous,  regularly-ramifying  struc- 
ture familiar  in  the  stems  of  the  ma  j  ority  of  plants .  Although  older 
stems  become  strongly  thickened  and  woody,  and  protectively 
enwrapped  in  layers  of  bark,  the  young  growth  is  soft  and  green 
like  the  leaf,  and  likewise  consists  of  veins  and  soft  tissue,  though 
the  relative  importance  of  the  two  is  reversed  in  the  stem  as  com- 
pared with  the  leaf.  The  veins  can  be  seen  by  the  eye  in  young 
stems  that  are  translucent  (e.  g.,  Balsam),  when  these  are  held 
to  the  light;  and  they  can  also  be  made  visible  through  the  tissue 
in  some  others  if  these  are  stood  with  their  cut  ends  in  a  deeply- 
colored  liquid.  And  they  can  always  be  seen  in  thin  sections  cut 
crosswise  of  the  stem,  as  well  illustrated  in  some  later  figures 
(73,  139,  B)  which  accompany  a  fuller  discussion  of  the  stem 
in  another  connection.  The  veins  form  a  ring  in  most  kinds 
of  young  stems,  though  in  some  they  are  scattered  about;  and 
wherever  they  branch  to  run  out  to  the  leaves  the  stem  is  commonly 
swollen  a  little,  and  oftentimes  lighter  in  color,  giving  origin  to 
the  so-called  nodes  separated  by  spaces  called  internodes,  which 
are  by  no  means  " joints,"  as  sometimes  described.  Outside 
the  ring  of  the  veins,  as  the  later  figures  73  and  141  show  very 
clearly,  the  soft  tissue  holds  chlorophyll,  and  thus  aids  the  leaves 
in  their  photosynthetic  function.  The  amount  of  such  work 
that  stems  can  do  must  in  fact  be  little;  but  the  plant  takes  ad- 
vantage, as  it  were,  of  every  bit  of  its  surface  exposed  to  the  light 
and  not  needed  for  other  uses,  even  including  such  parts  as  the 
stamens  and  pistil  of  the  flower,  to  spread  out  additional  chloro- 
phyll for  the  invaluable  photosynthesis. 

Stems,  as  a  rule,  grow  continuously  from  buds  at  their  tips, 


62  The  Living  Plant 

and  new  branches  from  buds  in  the  angles  between  stems  and 
leaves, — a  position  which  has  the  advantage  of  nearness  to  the 
manufactories  of  food.  This  brings  us  to  consider  the  causes  which 
determine  the  arrangement  of  leaves  on  the  stem,  a  curious  matter, 
scientifically  called  phyllotaxy,  and  once  discussed  more  commonly 
than  now  in  botanical  books.  Leaves  do  not  originate  on  the  stem 
at  hap-hazard,  as  may  seem  the  case  on  some  slender  branches, 
but  in  quite  definite  and  even  mathematical  order,  as  rosette- 
like  plants,  cones,  and  some  other  very  compact  structures  sug- 
gest. Two  primary  systems  of  leaf-arrangement  are  possible, 
and  occur.  The  simplest  is  the  opposite  (or  whorled]  system,  in 
which  two  leaves  stand  at  the  same  node  exactly  opposite  one 
another,  as  occurs  for  example  in  the  Mints,  (figure  16,  A},  in 
which  case  the  next  pairs  above  and  below  stand  at  right  angles 
and  thus  cover  the  space  left  by  the  first  set,  producing  four  vertical 
rows  often  in  remarkable  symmetry,  as  our  common  cultivated 
Coleus  illustrates.  This,  with  the  other  arrangements,  is  shown 
diagrammatically  in  figure  16,  where  the  reader  is  supposed  to 
look  down  from  above  on  the  stem,  which  is  imagined  to  be  tel- 
escoped, so  to  speak,  Chinese  lantern  fashion,  to  a  single  flat  plane, 
as  indeed  the  stems  actually  are  in  the  buds.  In  some  kinds, 
three  instead  of  two  leaves  stand  at  a  node,  or  four  or  five,  or 
more,  producing  a  regular  whorl,  but  in  all  such  cases,  illustrated 
for  instance  by  large  Lilies  (figure  16,  B),  the  leaves  in  a  whorl 
are  evenly  spaced  and  cover  the  breaks  in  the  whorls  above  and 
below.  This  is  the  system  prevalent  in  flowers,  for,  as  everyone 
will  recall,  the  whorl  of  sepals  covers  the  breaks  in  the  whorl  of 
petals,  with  a  similar  arrangement  in  stamens  and  carpels.  Thus 
much  for  the  opposite  or  whorled  system;  the  other  is  the  spiral, 
in  which  only  one  leaf  ever  stands  at  a  node,  while  the  one  on  the 
node  next  above  or  below  stands  part  way  around  the  stem, 
the  successive  leaves  falling  always  into  a  regularly-ascending 
spiral.  Now  this  space  around  the  stem  from  one  leaf  to  another  is 
a  definite  fraction  of  the  circumference; — in  some  plants  it  is  ^, 


The  Profound  Effect  on  the  Structure  of  Plants      63 


FIG.  16. — Diagrams  to  illustrate  the  principal  systems  of  leaf-arrangement,  as  they  would 
appear  from  above  if  the  stems  were  telescoped  to  one  plane.  The  rings  are  nodes, 
and  the  small  heavy  circles  are  leaf  bases.  Further  particulars  in  the  text. 


64  The  Living  Plant 

as  in  the  Elm  and  Grasses,  in  which  case  one  must  pass  once 
round  the  stem  and  cover  two  spaces  to  reach  a  leaf  over  the 
first  (figure  16,  C).  In  others,  (e.  g.,  the  Sedges),  the  fraction  is 
1/3,  and  a  spiral  drawn  through  the  bases  of  the  leaves  passes 
once  round  the  stem  and  across  three  spaces  to  reach  a  leaf  over  the 
first  (figure  16,  Z>).  In  others,  (e.  g.,  the  Apple)  it  is  2/5,  when  the 
spiral  must  pass  twice  around  the  stem  and  cross  five  spaces  to 
come  to  a  leaf  over  the  first  (figure  16,  E),  an  arrangement  which 
is,  perhaps,  the  commonest  of  all.  In  others  the  fraction  is  3/8 
(in  Holly  and  Plantain  figure  16,  F),  or  °/13,  as  in  cones  of  White 
Pine,  while  8/21,  13/34,  and  even  some  higher  fractions  are  said  to 
have  been  traced  in  special  places  where  the  leaves  are  greatly 
condensed  together  in  rosettes.  And  a  curious  thing  is  this,  that 
while  these  fractions  occur,  the  various  possible  intermediate 
ones  do  not.  In  these  fractions,  which  primarily  express  the 
amount  of  circumference  between  two  successive  leaves,  the 
numerator  also  expresses  the  number  of  turns  that  must  be  made 
around  the  stem  to  reach  a  leaf  over  the  first,  while  the  denomina- 
tor expresses  the  number  of  spaces  that  must  be  passed  over  for 
this  purpose,  and  also  the  number  of  vertical  ranks  into  which  the 
leaves  fall.  Moreover,  these  fractions  bear  to  one  another  a  very 
curious  relationship,  for  when  they  are  arranged  in  a  series, — viz., 

*/2,   Va,  2/5,  3/8,   5/13,  8/21,   13/34 

it  is  found  that  each  numerator  is  the  sum  of  the  two  numerators 
preceding,  and  each  denominator  likewise  the  sum  of  its  two  pre- 
decessors, and  moreover  each  numerator  is  the  same  as  the  de- 
nominator next  before  the  preceding.  This  curious  series,  known 
in  mathematics  as  the  Fibonacci  series,  is  said  to  find  expression 
in  other  phenomena  of  nature,  including  the  arrangement  of  the 
planets,  and  is  therefore  not  peculiar  to  the  phyllotaxy  of  plants. 
The  question  of  present  importance,  however,  is  this, — what  is  its 
meaning  in  connection  with  leaf-arrangement?  Of  course  one's 
first  natural  thought  is, — adaptation,  which  appears  reasonable 
enough  with  the  opposite  system  and  the  whorls,  and  even  with 


The  Profound  Effect  on  the  Structure  of  Plants      65 

the  lower  fractions  of  the  spiral  system,  where  one  can  see  the 
advantage  of  a  spacing  which  may  give  to  the  leaves  the  best 
aggregate  exposure  to  light.  But  this  interpretation  meets  in- 
creasing difficulties  with  the  higher  fractions,  and  even  has  trouble 
with  the  lower  when  one  notices  how  freely  the  leaf-blades,  the 
very  parts  which  need  the  exposure  to  light,  are  swung  by  their 
slender  petioles  into  positions  of  advantageous  individual  exposure 
in  callous  disregard  of  the  orderly  arrangement  in  which  they  start 
from  the  stem.  There  is,  however,  another  and  very  different 
explanation  of  the  systems  of  phyllotaxy  advanced  by  some  in- 
vestigators, viz.,  that  they  are  wholly  determined  by  the  positions 
in  which  the  young  leaves  originate  inside  of  the  growing  bud, 
which  positions  in  turn  are  determined  by  mechanical  principles 
connected  with  the  easiest  mode  of  origin  of  new  swelling  parts 
in  buds  of  a  certain  size  and  shape.  In  other  words  the  fractions 
of  phyllotaxy  are  merely  an  incidental  result  of  mechanical 
conditions  present  in  growing  buds,  and  have  only  a  secondary, 
if  any,  reference  to  adaptation.  This  explanation  I  believe  to  be 
substantially  correct.  It  is  of  course  not  an  explanation  of 
phyllotaxy,  but  merely  a  transference  of  the  problem  into  an- 
other field,  as  most  of  our  explanations  are.  But  I  dwell  upon  the 
subject  at  this  length  because  phyllotaxy  seems  to  me  to  offer  a 
fairly  clear  case  in  which  a  conspicuous  feature  of  plant  structure 
has  merely  an  incidental  and  not  an  adaptive  origin. 

There  is  one  other  feature  of  leaf  and  stem  structure  to  which  I 
have  not  yet  made  any  particular  reference,  and  that  concerns 
their  sizes,  which  are  wonderfully  diverse  in  different  plants. 
Leaves  are  measured  in  terms  of  feet  in  Bananas  and  Palms,  but 
need  the  assistance  of  lenses  to  show  them  at  all  in  some  of  the 
kinds  that  grow  in  the  deserts;  they  are  merely  of  tissue  thinness 
in  some  kinds  of  Ferns,  but  cylindrically-thick  and  stem-like  in 
Aloes  and  Century  Plants.  Stems  display  a  thousand  feet  of 
length  in  the  Rattan  Palm,  but  are  invisible  supports  to  tufts 
of  leaves  in  the  Houseleek;  nearly  as  thin  as  a  hair  in  some  Ferns, 


66  The  Living  Plant 

but  quite  as  thick  as  a  house  in  the  larger  species  of  Redwood; 
branched  to  a  spray  hi  a  Mango  Tree,  but  an  unbranched  shaft 
in  the  Royal  Palm.  Thus  it  is  evident  that  leaves  and  stems  ex- 
hibit well-nigh  as  remarkable  a  diversity  in  size  as  in  shape,  and 
we  must  conceive  of  our  generalized  or  composite  leaf  and  stem 
as  well-nigh  indefinitely  modifiable,  possessing,  as  it  were,  a 
kind  of  a  super-elasticity  in  both  of  these  features.  As  to  the 
causes  determining  size  in  these  parts,  that  is  reserved  for  dis- 
cussion in  the  chapter  on  Protection,  where  it  will  be  shown  that 
the  size  actually  displayed  by  any  leaf  or  stem  represents  in  the 
main  a  compromise  or  truce  between  the  conflicting  tendencies 
of  the  plant  to  make  its  leaves  larger  for  photosynthetic  advantage 
on  the  one  hand,  and  smaller  for  better  resistance  to  hostile  ex- 
ternal conditions  on  the  other. 

In  this  chapter  thus  far  but  little  has  been  said  concerning  the 
root.  This  is  because  the  consideration  of  that  organ  is  more 
convenient  and  natural  in  the  chapter  that  deals  with  its  function 
of  Absorption;  and  there  its  description  will  be  found  in  detail. 
It  is  enough  for  our  immediate  purpose  to  say  that  roots,  the 
principal  organs  for  the  absorption  of  water  and  minerals,  and 
the  third  of  the  primary  plant  parts,  grow  out  from  stems,  which 
they  closely  resemble  in  structure,  having  much  the  same  internal 
cellular  construction  as  well  as  the  same  long-tapering,  freely- 
branching  forms.  Though  not  without  diversity  in  form,  size, 
and  structure,  they  are  yet  far  less  varied  in  these  respects  than 
are  leaves  and  stems,  and  for  a  sufficient  and  obvious  reason, — 
namely,  they  grow  under  far  more  uniform  conditions;  for  life 
in  the  soil  is  much  the  same  thing  all  the  world  over,  however 
varied  it  may  be  upon  the  surface. 

Thus  far  we  have  considered  only  those  diversities  which  leaves 
and  stems  exhibit  while  still  retaining  their  typical  function  of 
photosynthesis.  But  their  remarkable  plasticity  does  not  exhaust 
itself  here,  for  these  parts  can  even  perform  entirely  different 
functions,  becoming  adaptively  modified  therefor  to  such  a  de- 


The  Profound  Effect  on  the  Structure  of  Plants      67 

gree  that  their  original  nature  would  hardly  be  suspected  were  it 
not  for  the  existence  of  intermediate  stages.  And  not  only  that, 
but  conversely,  substantially  all  of  the  structures  performing 
remarkable  or  unusual  functions  and  displaying  remarkable  forms, 
are  simply  transformations  of  the  three  primary  parts,  leaf,  stem 
and  root.  This  subject  of  the  formation  of  all  the  special  organs  of 
plants  out  of  leaf,  stem,  and  root,  (a  typical  example,  by  the  way, 
of  morphological  study,)  we  must  now  proceed  to  consider. 

The  particular  structures  performing  definite  functions  in  typical 
plants,  other  than  ordinary  leaf,  stem,  and  root,  are  the  following : 

Bud  coverings,  or  scales,  give  needed  protection  to  living  buds 
over  winter.  Adaptively  to  this  function,  they  are  small,  con- 
caved, thick,  corky,  brown,  and  often  resinous,  as  the  large  winter 
buds  of  any  common  trees  will  illustrate. 
Bud  scales  are  transformed  leaves,  usually 
leaf -blades,  but  in  some  plants  (e.  g.,  the 
Horse  Chestnut)  are  petioles,  the  blades 
being  suppressed,  while  in  others  they  are 
stipules,  as  shows  very  beautifully  in  the 
Tulip  Tree  (figure  17.) 

Tendrils,  or  similar  parts,  enable  slender 
plants  to  cling  to  a  support  and  thus  mount 
upward  towards  the  light.  Adaptively  to 
this  function  they  are  slender,  tough,  cy- 
lindrical, or  cord-like  structures,  endowed 

FIG.   17.— The    stipular    bud 

with  remarkable  powers  (to  be  later  con-     coverings  of  the  Tulip  Tree; 

•  j          i   .       ,1          i  T*J.I_*TJ.\        £       one-third  natural  size. 

sidered  in  the  chapter  on  Irritability),  ot 

reaching  out  for  a  support,  taking  a  firm  hold  thereon,  and  sub- 
sequently shortening  and  toughening  then-  structure  (figure  85). 
The  best  tendrils,  like  those  of  the  Passion  Vine  or  the  Grape,  are 
transformed  stems,  issuing  from  buds  precisely  as  branches  do. 
Others  are  transformed  leaf-blades,  as  in  the  curious  Lathyrus 
Aphaca  (figure  18),  or  a  part  thereof,  as  in  Vetches,  or  Bignonia; 
or  are  stipules,  as  in  the  Wild  Smilax,  or  merely  the  petiole 


68 


The  Living  Plant 


which  makes  a  turn  around  some  object,  as  in  the  Clematis,  or 
a  cylindrical  part  between  two  portions  of  blades  as  in  those 
Pitcher  plants  called  Nepenthes  (figure  20).  In  some  tropical 
plants,  e.  g.,  climbing  Aroids,  the  aerial 
roots  clasp  horizontally  around  a  support. 
In  some  others,  and  notably  those  having 
the  habit  of  the  Ivies,  and  growing  against 
stonework,  the  tips  of  the  tendrils  do  not 
twine  around  a  support,  but  end  in  discs 
which  are  firmly  appressed  to  the  stones, 
as  in  the  Woodbine,  though  more  com- 
monly the  disc-holding  structures  are  aerial 
roots,  as  the  English  Ivy  illustrates. 

ProJect   rePeUmgly   from   some 


FIG.    is.-Tendriis    trans- 
formed   from  leaf-blades,  kinds  of  plants  as  if  they  might  form  a 

with     stipular    foliage,    of  .  ,  i  r     i 

Lathyrus  Aphaca;  one-half  protection  against    the   attacks  of   large 
plant-eating  beasts.    They  possess  a  stiff, 

hard,  conical  structure,  and  a  firm  attachment  to  the  skeleton, 
consistent  with  that  use.  In  some  plants  they  are  no  more  than 
prickles,  erupted,  so  to  speak,  from  the  surface,  as  in  the  Rose; 
in  other  cases  they  are  the  sharp- 
ened ends  of  the  veins,  as  in  the 
Holly;  in  others  they  are  the  leaf- 
blades,  as  in  the  Barberry  and 
the  Cactus;  in  others  they  are 
stipules  as  in  the  most  spiny 
of  the  Euphorbias  (figure  19), 

though  in  SOme    Other   kinds  the    FIG.  19.—  The  stipular  spines  of  Euphorbia 

spines  are  the  persistent  and  in- 

durated floral  branches;  in  others,  such  as  the  Locusts,  they  are 

transformed  branches  coming  from  ordinary  axillary  buds;  in 

some  Palms  they  are  roots  ;  and  cases  are  known  where  they  are 

petioles. 

Food  Reservoirs  store  up  for  later  use  the  food-material  made 


The  Profound  Effect  on  the  Structure  of  Plants      69 

in  the  leaves  of  herbaceous  perennial  plants,  and,  adapt  ively 
to  this  function,  are  greatly-swollen,  soft-bodied,  large-cellular 
structures.  They  are  leaves  in  the  bulb  scales  of  Lilies  and  Hya- 
cinths, stems  in  the  common  Potato  (the  eyes  being  axillary 
buds),  and  roots  in  the  Sweet  Potato. 

Insect  Traps  effect  the  capture  and  digestion  of  insects,  and 
thus  enable  some  plants  to  augment  the  scanty  supply  of  nitrog- 
enous compounds  available  where  they 
grow.  Adaptively  thereto  these  traps  have 
highly  special  forms  and  accessory  features 
contributing  to  the  attraction  and  capture 
of  insects,  as  will  later  be  noted  in  a  par- 
ticular description  of  these  plants.  The 
trap  is  a  pitcher  formed  by  a  special  cup- 
like-upgrowth  of  the  leaf-blade,  as  in  the 
various  Pitcher  Plants  (figure  20),  or  else 
a  hinged  or  inrolling  blade,  as  in  the  Venus 
Fly-trap  and  Sundew. 

Flower  parts  contribute  in  various  ways 
to  the  efficiency  of  reproduction,  as  will 
later  appear  in  a  discussion  of  that  subject. 
The  parts  are  transformed  leaves,  and  dis- 
play features  adaptive  to  their  functions,  — 
the  green  leaf-like  sepals  which  protect 
the  other  parts  while  in  bud,  the  brightly- 
colored  petals  which  exhibit  the  position  of 
the  flower  to  the  visiting  insect,  and  (though 
with  a  reservation)  the  stamens  and  pistil  FlG.  20.—  An  insect-trap- 
concerned  with  the  actual  pollination.  In 


SOme  kinds   Of   flowers   the  petals  are  miss-        leaf  tip  in  Nepenthes;  one- 

third  natural  size. 

ing,   but   their  function  is   performed  by 

brilliantly-colored  leaves  close  under  the  flowers,  as  shown  so 

strikingly  in  the  Poinsettia. 

Miscellaneous.  There  are,  furthermore,  a  great  many  special 


The  Living  Plant 


structures  with  particular  functions  not  belonging  in  any  of  the 
definite  categories  above  mentioned.  Thus,  the  bladdery  air- 
filled  floats  which  keep  the  Water  Hyacinth  resting  so  lightly 
on  the  water  are  petioles;  the  wing  which  ensures  the  carriage 
of  the  Linden  seeds  is  a  leaf-blade  (figure  157) ;  the  indurated  hooks 
by  which  some  tropical  vines  do  their  climbing  are  stipules ;  while 
the  reduced  or  rudimentary  leaves  which  we  call  bracts  often 
also  possess  functions  of  a  minor  sort. 

Substitution  foliage.  Finally,  we  must  take  notice  of  another  curi- 
ous transformation  in  function  and  structure  found  in  all  parts 
other  than  the  leaf-blade,  namely,  they  may  be- 
come transformed  into  foliage,  either  in  aid  of  the 
blade,  or  its  replacement.  Thus,  in  some  kinds, 
the  blade  is  greatly  reduced  or  missing,  and  the 
petiole  is  flattened  and  thin  and  acts  as  the  foliage, 
e.  g.  in  the  Australian  Acacias  (figure  21),  and 
some  kinds  of  Oxalis.  In  a  good  many  plants 
the  stipules  are  sufficiently  big  to  render  appreci- 
able aid  to  the  leaf-blade.  In  Lathyrus  Aphaca 
(figure  18)  they  form  all  of  the  foliage  there  is, 
while  in  the  common  Bedstraw  or  Galium,  they 
are  as  large  as  the  leaves  and  so  like  them  as 
F7ened  petiole  se™~  commonly  to  be  thought  additional  leaves  helping 
??  ut8,i  f°H?ge  to  make  up  a  whorl.  In  a  great  many  plants, 

(the  hlnuc.s  being 

insignificant),   in  and  especially  those  found  in  dry  places,  the  leaves 
Acacia;   one-half  become  very  small  or  are  absent,  and  the  function 
of  foliage  is  performed  by  the  stem,  which  either 
remains  smooth  and  round,  or  becomes  fluted  by  the  presence  of 
vertical  green  ribs,  or  becomes  flattened  in  various  degrees,  all 
three  conditions  of  which  are  found  in  the  family  of  Cactuses.    In 
some  cases  the  stem  is  flattened  as  thin  as  a  leaf,  while  still  dis- 
playing the  nodes  distinctive  of  the  stem,  as  in  the  Muehlenbeckia 
of  our  greenhouses  (figure  22) ;  but  in  other  cases  no  nodes  appear, 
and  the  stem  assumes  a  form  and  general  aspect  so  leaf-like  that 


The  Profound  Effect  on  the  Structure  of  Plants      71 

the  botanical  teacher  has  often  much  ado  to  convince  his  students 
that  it  is  anything  else,  even  when  he  shows  them  the  actual 
leaves,  reduced  to  scaly  bracts,  out  of  whose  axils  the  leaf-like 
branches  clearly  spring.  Such  is  the  case  with  the  Butcher's 
Broom  of  Europe,  (figure  23),  our  common  Asparagus,  and  the 
cultivated  Smilax  of  the  florists.  Finally  there  is  even  a  case 


FIG.  22. — The  leaf-like  stem,    with    some    small    leaves,    of    Muehlenbeckia;    one-half 

natural  size. 
FIG.  23. — The  leaf-like  branches  of  Butcher's  Broom;  one-half  natural  size. 

in  a  tropical  Orchid,  Taeniophyllum  by  name,  where  the  roots 
serve  as  foliage,  becoming  suitably  flattened  and  otherwise  ap- 
propriately constructed. 

We  cannot  take  space  to  follow  any  farther  this  most  interest- 
ing subject,  but  if  the  reader  desires  another  and  much  fuller 
discussion  thereof,  he  will  find  it  in  the  appropriate  places  hi  Asa 
Gray's  Structural  Botany,  where  it  is  treated  in  a  manner  that  in 
my  opinion  cannot  be  surpassed.  The  subject,  moreover,  is  one 
which  offers  attractive  opportunity  for  concentrated  field  study 


VUO^WW^l  0^  V\VT(\\>mQ  vJra 


Fio.  24. — A  collection  of  specimens,  pressed  and  dried,  and  arranged  to  illustrate 
morphological  topic ;  photographed  one-third  the  original  size. 


The  Profound  Effect  on  the  Structure  of  Plants      73 

in  the  discovery,  identification,  collection  and  arrangement  of  the 
various  special  structures  of  plants,  which  can  then  be  preserved 
in  some  such  manner  as  our  picture  illustrates  (figure  24). 

Thus  it  is  evident  that,  on  the  one  hand,  the  three  primary 
plant  parts, — leaf,  stem  and  root, — though  developed  with  a 
structure  adaptive  to  the  very  particular  function  of  photo- 
synthesis or  food-making,  have  in  many  cases  become  trans- 
formed into  other  parts  of  very  different  ecological  significance 
and  structure;  while,  on  the  other  hand,  and  correlatively,  all 
of  the  great  number  of  highly  specialized  parts  performing  other 
functions  can  be  traced  back  to  an  origin  morphologically  in  the 
three  primary  plant  parts.  This  interlocking  relationship  of 
morphological  origin  with  ecological  meaning, — of  morphology 
with  ecology, — can  perhaps  be  made  clearer  by  use  of  a  diagram 
such  as  is  given  herewith  (figure  25). 

Although  I  ought  now  to  end  this  long  chapter,  I  will  continue 
far  enough  to  answer  two  questions  which  I  am  sure  have  arisen 
in  the  mind  of  the  reader.  Thus,  he  will  surely  be  wondering 
why  it  is  that  some  plants  make  their  tendrils,  for  instance,  from 
leaf-blades,  others  from  petioles,  others  from  stipules,  others  from 
stems,  and  others  even  from  roots.  The  most  reasonable  answer 
appears  to  be  this,  that  when  a  plant,  owing  to  a  change  of  habit 
forced  on  it  by  a  change  of  environment,  develops  a  need  for  a  new 
organ,  that  organ  is  made  by  a  transformation  of  the  part  which 
happens  to  be  most  available  for  the  purpose,  often  some  part 
which  the  change  of  habit  has  happened  to  set  free  from  its 
former  use;  and  sometimes  that  most  available  part  will  be  one 
thing  and  sometimes  another.  In  the  second  place  the  reader 
will  wonder  why  some  plants  should  abandon  their  leaf-blades 
as  foliage,  and  then  proceed  to  replace  them  by  petioles,  stipules, 
stems,  or  even  roots,  which  are  for  the  purpose  converted  physi- 
ologically and  structurally  into  leaves.  In  answer  it  may  be  said 
that  the  abandonment  of  the  leaf-blade,  as  will  be  shown  in  the 
chapter  on  Protection,  usually  accompanies  exposure  to  very  dry 


74 


The  Living  Plant 


Leaf-blades 


Petioles 


Flower  parts 


Foliage 


Insect  traps 


Bud  covers 


Stipules 


Stems 


Roots 


Storage 


Absorption 


Fio.  25. — Diagram  to  illustrate  the  interrelations  of  morphological  origins  with  ecological 
uses  in  the  parts  of  the  higher  plants. 

climate,  in  which  case  the  function  of  foliage  is  taken  over  by 
some  other  part,  usually  the  stem.  Now  it  is  conceivable  that 
when,  by  another  change  of  habit,  the  plant  finds  itself  in  need 
of  a  much  larger  spread  of  chlorophyll  surface,  this  may  be  more 
easily  obtained  by  further  enlarging  and  flattening  the  already 


The  Profound  Effect  on  the  Structure  of  Plants      75 

leaf -like  stem  than  by  re-developing  the  lost  leaves.  It  is  probable 
that  some  peculiarity  of  this  kind  in  the  past  history  of  the  plant 
will  explain  in  each  case  such  curious  features,  the  course  of  devel- 
opment being  always  that  which  offers  the  least  resistance  at  the 
moment. 

The  reader  will  now  be  prepared,  I  think,  to  admit  that  of  all 
the  influences  concerned  in  the  determination  of  plant  form, — 
indeed  in  making  plants  what  they  are, — the  most  important  by 
far  is  the  physiological  process  of  food-making,  or  photosynthesis, 
and  that  the  feature  of  this  process  having  the  most  profound 
effect  is  the  need  for  exposure  to  light. 


CHAPTER  IV 

THE  KINDS  OF  WORK  THAT  ARE  DONE  BY  PLANTS,  AND 
THE  SOURCE  OF  THEIR  POWER  TO  DO  IT 

Respiration 


HEN  first  I  had  written  this  chapter,  and  made  it  the 
best  that  I  could,  it  assumed  that  the  fact  of  plant 
work  was  already  well-known  to  the  reader.  A  later 
experience,  however,  made  me  see  very  clearly  that 
most  people  do  not  know  that  plants  work  at  all.  Accordingly 
I  shall  make  it  my  first  endeavor  to  show  beyond  question  that 
plants  do  work;  then  we  can  pass  with  better  understanding  to 
the  study  of  the  very  remarkable  source  from  which  they  derive 
their  power  to  do  it. 

The  principal  reason  why  the  majority  of  people  do  not  as- 
sociate with  plants  the  idea  of  work  is  found  in  the  slowness  of 
most  plant  actions.  Our  conception  of  work  is  almost  entirely 
subjective,  and  because  plants  are  placid  of  mien,  and  do  not  hurry 
and  fret  and  strain,  we  think  they  are  doing  no  work.  When  the 
Master  said  of  the  Lilies,  that  they  toil  not  neither  do  they  spin, 
his  words  expressed  the  popular  fancy  but  not  the  physical  fact. 
Work  is  none  the  less  real  because  it  is  slow,  and  the  matter  of 
slowness  is  entirely  relative  and  subjective.  Even  the  very  swift- 
est actions  performed  by  any  of  us  must  seem  slowness  person- 
ified to  the  lightning,  or  to  a  dynamite  charge  which  can  finish 
its  work  before  you  can  think,  or  to  the  forces  of  collision  which 
reduce  a  railway  train  to  a  heap  of  tangled  scraps  within  the 
space  of  an  instant.  Probably  the  lightning,  the  dynamite,  or 
the  collision  forces,  if  interviewed  on  the  subject,  would  say  that 

76 


The  Kinds  of  Work  That  Are  Done  by  Plants       77 

mankind  does  not  work.  But  if  plant  actions  could  be  magnified 
immensely  in  speed  they  would  impress  one  very  differently  in 
this  particular.  For  then  the  observer  would  see  the  tip  of  every 
growing  plant-structure  nodding  and  moving  energetically  about, 
so  that  a  meadow,  a  copse,  or  a  forest  would  seem  all  of  a  vigor- 
ous tremble  as  if  straining  at  some  hidden  leash :  he  would  see  the 
buds  of  some  flowers  open  and  close  with  a  straining  yawn  or 
a  sudden  snap,  and  others  burst  into  bloom  like  a  rocket  when  it 
breaks  to  a  spray  of  mani-colored  lights:  roots  in  their  efforts 
to  penetrate  the  earth  turning  and  twisting  like  angleworms  im- 
paled on  the  fisherman's  hook:  seedlings  in  their  struggle  to  break 
through  the  ground  heaving  arid  straining  at  their  burden  of 
superincumbent  soil,  like  a  powerful  man  at  some  load  which 
has  fallen  upon  him :  seed  pods  pushing  into  the  earth  on  a  twist- 
ing or  hard-thrust  stalk :  tendrils  swooping  in  curves  through  the 
air,  gripping  the  first  thing  they  meet,  and  jerking  their  plants 
towards  the  support.  As  matter  of  fact,  there  does  exist  a  way 
in  which  we  can  readily  behold  these  actions  thus  magnified, 
for  if  the  structure  in  question  be  photographed  at  regular  inter- 
vals, say  of  fifteen  minutes  to  half  an  hour,  and  then  these  photo- 
graphs are  run  at  high  speed  through  a  moving-picture  machine, — 
the  thing  is  done.  Such  studies  have  actually  been  made  in  the 
case  of  twisting  roots,  moving  fruits,  and  opening  flowers;  and  all 
of  those  who  have  seen  them  agree  in  the  impression  of  vigorous 
work  thus  presented. 

Furthermore,  if  we  could  magnify  in  like  manner  the  interior 
parts  of  the  plant  we  should  witness  as  remarkable  actions  pro- 
ceeding with  equivalent  vigor.  In  some  plants  the  living  proto- 
plasm would  be  seen  flowing  in  thick  turbid  streams  round  and 
round  within  the  encasing  cell- wall;  in  certain  cells  those  re- 
markable structures  called  chromosomes  would  be  seen  perform- 
ing their  curious  manoeuvres, — arranging  themselves  into  groups, 
collecting  in  pairs,  passing  backward  and  forward  in  a  manner 
suggestive  of  the  measures  of  the  dancers  in  a  quadrille;  else- 


78  The  Living  Plant 

where  new  cells  would  be  seen  in  process  of  birth,  and  engaged  in 
forcing  the  older  apart  to  make  room  for  themselves;  while  minor 
actions  without  number,  mechanical,  physical,  and  chemical, 
would  appear  in  vigorous  progress  in  various  parts  of  the  organ- 
ism. Truly  if  one  could  see  these  actions  under  the  conditions 
here  imagined,  he  would  have  no  trouble  at  all  in  connecting 
with  plants  the  idea  of  real  work. 

We  are  not,  however,  dependent  solely  on  imagination,  or 
the  moving-picture  machine,  for  a  conception  of  the  reality  of 
plant  work.  The  rapid  closing  of  the  leaf  of  a  Venus  Fly-trap 
upon  a  captured  insect,  or  the  sudden  collapse  of  the  Sensitive 
Plant  when  touched,  suggest  some  such  idea.  Everybody  has 
noticed  that  the  great  granite  curbstones  along  streets  where 
shade  trees  are  grown,  become  heaved  from  the  regular  lines  in 
which  they  are  laid,  while  the  pavements  themselves .  are  often- 
times thrown  into  irregular  swells;  this  is  all  brought  about  by 
the  growth  of  the  roots  of  the  trees,  which  thus  exhibit  a  work  as 
real  as  that  of  a  jack-screw  or  derrick.  If  the  reader  has  not  al- 
ready observed  these  phenomena,  let  him  do  so  when  next  he 
walks  through  a  shaded  street.  In  a  similar  manner  young  roots, 
insinuated  between  the  stones  of  buildings,  tombs,  or  walls, 
force  the  masonry  apart  in  their  growth,  and  finally  accomplish 
the  destruction  of  the  edifice.  Occasionally  asphalt  pavements 
are  burst  upwards  by  the  growth  of  some  kinds  of  plants,  including 
even  soft-bodied  Fungi,  as  the  accompanying  photograph  well 
proves  (figure  26).  And  the  technical  literature  of  plant  physi- 
ology tells  of  the  thousands  of  pounds  pressure  exerted  by  large 
gourds,  like  Squash,  when  suitably  harnessed  to  recording  machin- 
ery. And,  finally,  experiment  proves  that  every  operation  of 
plant  life,  even  the  least  of  them  all,  involves  some  movement, 
and  therefore  real  work;  so  that  animals  and  plants  are  working, 
and  often  right  hard  from  the  physical  point  of  view,  when  they 
merely  are  keeping  alive, — a  conclusion  from  which  the  reader 
is  welcome  to  draw  any  comfort  that  he  can. 


The  Kinds  of  Work  That  Are  Done  by  Plants        79 

At  this  point,  perhaps,  some  one  will  rise  and  declare  I  am  wrong 
in  my  statement  that  work  is  as  real  when  slow  as  when  swift. 
But  note  that  I  say  as  real,  not  as  hard.  When  a  weight  of  a  ton 
is  lifted  a  foot,  no  matter  by  what  means,  the  work  is  the  same 
whether  done  in  a  day  or  a  minute,  although  it  is  over  a  thousand 
times  harder  to  do,  (to  be  exact,  the  power  required,  is  1440  times 
greater)  in  the  latter  case  than  the  former.  But  the  fact  of  im- 


FIG.  26. — An  asphalt  pavement  burst  upward  by  the  growth  of  soft-bodied  mushrooms, 
whose  conical  heads  are  visible  over  the  wreckage. 

mediate  importance  is  this,  that  the  work  is  as  real  in  one  case 
as  the  other. 

We  come  now  to  the  bond  of  connection  between  this  matter 
of  plant  work  and  the  principal  theme  of  this  chapter,  viz., — it 
is  a  fact  of  physics,  which  the  reader  must  long  since  have  learned, 
that  every  bit  of  work  of  every  kind  done  anywhere  whatsoever 
in  nature,  whether  in  a  plant,  or  an  engine,  or  the  skies,  or  the 
thinking  brain  of  a  man,  requires  for  its  accomplishment  the 
presence  and  expenditure  of  energy,  which  is  the  source  of  all 
power.  The  reader,  of  course,  knows  what  energy  is, — the  en- 
tity in  Nature,  and  the  only  one,  that  produces  motion  by  which 


8o  The  Living  Plant 

work  is  accomplished.  Energy  is  most  familiar  as  heat  or  elec- 
tricity, though  manifest  also  in  light  and  in  chemical  reactions. 
Without  energy  there  is  no  motion,  no  power,  no  work;  and  with- 
out it  a  plant  or  an  animal  stops  as  dead  as  an  engine  when  no  fire 
burns  under  its  boiler.  Plant  work,  therefore,  requires  and  im- 
plies a  supply  of  energy.  And  with  this  conclusion  it  will  be  well 
to  gather  the  foregoing  matters  into  a  generalization,  another 
of  our  botanical  verities; — all  plants,  like  all  animals,  are  inces- 
santly at  work  while  alive,  as  truly  as  any  moving  machine,  not  only  in 
the  performance  of  their  active  and  visible  movements,  but  also  in  the 
bare  maintenance  of  their  existence;  and  this  work  requires  a  pro~ 
portional  supply  of  energy. 

It  is  now  our  business  to  find  the  source  of  the  energy  by  which 
plants  do  their  work.  We  know  the  source  of  the  energy  in  the 
work  of  the  engine  just  mentioned;  it  is  the  heat  released  from 
the  burning  of  coal  in  a  grate.  But  what  is  the  source  of  the  energy 
in  the  work  of  the  plant,  which  has  neither  grates,  nor  boilers, 
nor  flaming  of  fuel? 

When  the  student  of  science  is  faced  by  a  problem  like  this, 
his  first  resource  is  to  look  around  for  suggestions  from  some 
analogous  process.  In  this  instance  he  would  turn  naturally 
to  animals,  and  his  earlier  studies  on  the  physiology  of  man  would  . 
have  taught  him  that  the  power  of  animals  to  do  work  is  connected 
in  some  way  with  their  respiration, — that  process  in  which  they 
give  forth  the  gases  carbon  dioxide  and  water  vapor  to  the  air, 
while  absorbing  the  gas  oxygen  into  their  bodies.  How  inti- 
mately this  process  is  connected  with  work  is  easily  realized 
when  we  recall  the  familiar  fact  that  respiration  increases  in  pro- 
portion as  work  becomes  harder.  Is  it  possible,  then,  that 
plants  also  respire?  That  is,  do  plants  in  their  work  release  car- 
bon dioxide,  and  absorb  oxygen?  Obviously  this  matter  is  de- 
terminable  by  experiment,  and  the  following  is  a  very  good 
method.  In  a  bottle  arranged  as  shown  by  the  picture  (figure 
27),  we  place  some  plant  parts  which  are  actively  working  with- 


The  Kinds  of  Work  That  Are  Done  by  Plants       81 


out  the  complications  introduced  by  photosynthesis  (e.  g.,  ger- 
minating seeds,  such  as  Oats),  then  close  the  bottle  air-tight  by 
means  of  the  stoppers  and  clamp  provided  for  the  purpose,  and 
stand  it  for  some  hours  in  a  warm 
and  dark  place  where  growth  can 
take  place.  Obviously,  any  carbon 
dioxide  released  by  the  seeds  must 
collect  in  the  bottle,  where  its  pres- 
ence may  be  detected  by  its  well- 
known  property  of  turning  clear  lime- 
water  milky.  If,  accordingly,  clear 
limewater  is  poured  into  the  tall  vessel 
into  which  the  delivery  tube  leads, 
the  clamp  is  loosened,  and  water  is 
poured  down  the  thistle  tube,  then 
the  gas  will  be  forced  from  the  bottle 
and  sent  bubbling  up  through  the 
limewater.  The  result  is  always  de- 
cisive. The  limewater  turns  white- 
milky  proving  the  presence  of  car- 
bon dioxide  hi  abundance.  And  if 
a  bright  person  should  here  rise  to 
remark  that  the  carbon  dioxide  al- 
ways present  in  air  is  sufficient  to  ex-  FIG.  27.— A  Respiroscope,  or  ar- 

...  .  rangement  for  demonstrating  that 

plain  the  result,  it  IS  easy  tO  prove  it        plants    respire.     Its    operation  is 

is  not;  for,  if  an  equal  quantity  of  air  explained  in  the  text' 
be  forced  from  an  empty  bottle  through  limewater  no  milkiness 
appears.  Arid  if,  in  the  bottle,  we  place  buds,  or  roots,  or  color- 
less plants  like  Mushrooms,  or  even  green  leaves  (in  the  dark),  the 
result  is  always  the  same.  Furthermore,  it  is  also  the  same  whether 
the  working  parts  are  kept  in  the  light  or  the  dark,  and  it  is  still 
the  same,  as  the  reader  may  be  confounded  to  learn,  even  with 
green  leaves  when  kept  in  the  light,  though  here  the  process  is 
obscured  by  the  absorption  of  that  gas  in  photosynthesis,  as  can 


5 

. 

1 
-, 

<i 

, 

i- 

82 


The  Living  Plant 


be  proven  by  experiments,  too  elaborate,  however,  for  description 
at  this  place.  Furthermore,  as  we  may  conveniently  note  here, 
all  of  these  same  working  parts  are  simultaneously  releasing  water 
as  well.  It  is  therefore  true,  as  a  general  principle,  that  all  working 

parts  of  all  plants  are 
giving  off  carbon  dioxide 
as  well  as  water,  pre- 
cisely as  animals  are  do- 
ing. 

But  do  plants  exhibit 
the  other  phenomenon 
of  animal  -respiration, — 
absorption  of  oxygen? 
It  is  very  easy  to  prove 
that  plants  must  have 
oxygen  in  order  to  live 
and  work,  precisely  as 
animals  must ;  for  if  two 
sets  of  the  same  seeds  are 
placed  in  two  similar 
closed  chambers,  and 
then  the  oxygen  is  re- 
moved from  one  chamber 
by  a  chemical  absorbent 
while  it  is  left  untouched 
in  the  other,  the  seeds  in 

I1  IG.  28. — Two  similar  tube-cham tiers  in  which  were 

placed  similar  sets  of  germinating  oats  kept  wet  the    OXVgenleSS    chamber 

and  in   place  by  wads  of  moss,   and  treated  pre-  . 

cisely  alike  except  that  those  on  the  right  were  de-  Will  not  germinate   at  all 

and  will  soon  die,  while  in 

the  other  they  will  grow  normally  for  a  considerable  time  (figure 
28).  Furthermore,  if  the  air  of  a  closed  chamber  in  which  seeds 
have  been  growing  for  some  days  be  subjected  to  chemical 
analysis,  it  is  found  that  most  of  the  oxygen  has  disappeared 
from  the  chamber,  and  must  therefore  have  been  absorbed  by 


The  Kinds  of  Work  That  Are  Done  by  Plants      83 

the  seeds.  And  the  same  thing  is  true  no  matter  what  structures 
we  place  in  the  chamber  (saving  only  an  apparent  exception, 
soon  to  be  noted,  in  the  case  of  lighted  green  leaves),  and  no 
matter  whether  the  chamber  is  exposed  to  the  light  or  kept  in 
the  dark.  It  is  evident,  therefore,  that  all  parts  of  working, 
(and  that  is  to  say,  of  living)  plants,  absorb  oxygen  and  release 
carbon  dioxide  precisely  as  animals  do. 

There  is  no  one,  I  think,  who  can  grasp  fully  the  bearings  of  a 
complicated  subject  after  only  a  single  presentation,  no  matter 
how  clear  this  may  be.  It  is  therefore  quite  likely  that  some  reader 
ere  this  has  experienced  a  feeling  of  dazement,  and  been  led  to 
exclaim,  along  with  the  much-puzzled  German,  "Jemand  ist 
verriickt,  aber  wer?";  and  he  may  even  incline  to  imagine  that 
I  am  the  "wer."  For  have  not  I  shown,  in  an  earlier  elaborate 
chapter,  that  plants  absorb  carbon  dioxide  and  release  oxygen, 
while  now  I  have  proven  by  evidence  quite  as  conclusive  that 
they  do  exactly  the  opposite?  But  there  is,  nevertheless,  no  in- 
consistency. For  the  reader  will  recall  that  it  is  only  the  green 
tissues  which  absorb  carbon  dioxide  and  release  oxygen,  and  then 
only  in  light,  and  then  only  from  the  tiny  little  chlorophyll  grains 
embedded  inside  of  the  protoplasm.  There  should  therefore  be 
no  trouble  in  understanding  how  the  protoplasm  in  which  those 
grains  are  embedded,  like  all  other  living  parts  of  the  plant,  can 
be  respiring,  while  the  chlorophyll  grains  alone  are  engaged  in  the 
photosynthetic  process.  The  case  of  the  chlorophyll  grains, 
however,  is  not  so  simple  as  my  statement  implies,  because, 
since  they  are  living  protoplasm,  there  is  every  reason  to  think 
that  they  also  respire  even  in  light,  and  that  hi  them, — and  in 
them  alone, — the  two  processes  go  on  together.  If,  now,  photo- 
synthesis and  respiration,  with  their  exactly  opposite  gas  ex- 
changes, proceed  together  in  leaves,  why  do  they  not  neutralize 
one  another's  results?  The  answer  is  easy.  Experiment  shows 
that  on  the  average  the  photosynthesis  in  green  leaves  in  the 
light  is  over  twelve  times  as  active  as  respiration  (and  it  may  rise 


84  The  Living  Plant 

very  much  higher),  a  preponderance  that  is  obviously  so  great 
as  to  over-balance  not  only  the  respiration  of  the  leaves,  but  of  all 
the  remainder  of  the  plant  besides,  and  not  for  daytime  alone, 
but  also  for  night.  Therefore,  day  and  night  together,  the  green 
plant  absorbs  much  more  carbon  dioxide  than  it  releases  and  re- 
leases much  more  oxygen  than  it  absorbs.  It  vitiates  the  air  by 
its  respiration,  but  in  the  long  run  purifies  it  still  more  by  its 
photosynthesis. 

Before  leaving  this  part  of  our  subject,  we  should  look  a  little 
more  closely  into  the  relations  of  the  two  processes  within  the 


FIG.  29. — Diagrammatic  sections  across  leaves,  to  illustrate  the  movements  of  gases  in 
and  out  of  the  same  during, — a,  light,  c,  darkness,  and  b,  the  balance  period  between. 
The  squares  are  carbon  dioxide,  the  triangles  are  oxygen,  and  the  arrows  show  the 
direction  of  movement. 

lighted  green  leaf, — a  subject  diagrammatically  illustrated  by  the 
accompanying  figures  (figure  29).  At  night  all  of  the  carbon 
dioxide  given  off  by  the  respiration  of  the  living  cells  into  the  air 
passages,  makes  its  way  along  these  and  through  the  stomata 
to  the  atmosphere  outside,  (figure  29,  c).  In  the  daytime  any 
carbon  dioxide  given  off  by  the  respiration  of  the  protoplasm  is 
absorbed  by  the  chlorophyll  grains  in  the  same  cells,  but  as  this 
supply  is  wholly  insufficient,  a  constant  stream  of  that  gas  passes 
in  from  the  atmosphere  through  the  stomata  and  along  the  pas- 
sages to  the  different  cells,  where  it  is  absorbed  by  the  chlorophyll 
grains;  simultaneously  a  part  of  the  oxygen  given  off  by  the 
chlorophyll  grains  is  absorbed  by  the  protoplasm  of  the  same  cells 
for  their  respiration,  while  the  very  large  surplus  is  sent  into  the 


The  Kinds  of  Work  That  Are  Done  by  Plants      85 

air  passages  and  along  them  and  through  the  stomata  to  the  at- 
mosphere; and  the  reader  should  thus  visualize  these  matters  in  his 
imagination  (figure  29,  a).  But  here  comes  an  interesting  point. 
Since  photosynthesis  is  dependent  upon  light  while  respiration  is 
not,  there  must  evidently  exist  a  certain  intensity  of  light  at  which 
the  two  processes  in  a  leaf  exactly  balance.  At  such  times  the 
processes  use  one  another's  gases,  and  there  is  no  movement  of 
carbon  dioxide  or  of  oxygen  either  into  or  out  of  the  leaf  (figure 
29,  6).  Such  a  balance  period  must  occur  every  day  just  after  sun- 
rise and  before  sunset,  and  on  some  very  dark  days  it  probably 
lasts  for  considerable  periods.  It  is  of  course  by  virtue  of  approx- 
imation to  such  a  balance  that  some  kinds  of  plants  such  as  Ferns, 
if  not  given  too  much  light,  can  thrive  so  well  for  long  periods 
of  time  in  tightly-closed  cases,  or  masses  of  red-berried  vines 
(Partridge-berry)  can  exist  all  winter  in  little  closed  globes  on 
dining-room  tables. 

We  may  now  express  the  important  facts  of  the  past  few  pages 
in  another  of  our  botanical  verities,  to  this  effect, — that  plants, 
like  animals,  respire,  and  in  identical  manner,  absorbing  oxygen 
and  releasing  carbon  dioxide,  throughout  all  of  their  living  parts. 

In  the  preceding  paragraph  I  have  said  that  the  gases  enter 
through  stomata  and  pass  along  air  passages,  but  I  have  given 
no  hint  of  the  forces  which  impel  them.  This  matter  will  be  taken 
up  fully  in  the  chapter  on  Absorption,  where  it  will  be  shown 
that  the  gases  move  along  diffusively  under  action  of  forces 
internal  to  themselves.  We  need  only  note  here  that  plants  have 
no  system  at  all  for  absorbing  and  expelling  large  masses  of  air 
as  animals  do  by  the  use  of  their  chest-muscles  and  lungs, — an 
operation  that  is  always  called  breathing.  Accordingly,  the  matter 
can  be  stated  in  this  way, — that  plants  respire,  but  do  not  breathe. 

It  will  be  well,  at  this  point,  to  turn  aside  for  a  moment  from 
our  main  subject  to  consider  some  phases  of  plant  respiration 
which  have  economic  importance.  The  first  is  concerned  with 
aeration  of  soils.  Roots,  like  all  other  living  parts,  must  respire 


86 


The  Living  Plant 


in  order  to  grow,  and,  with  the  exception  of  a  few  which  possess 
long  air  passages  connecting  with  the  leaves,  they  take  the  in- 
dispensable oxygen  from  air  in  the  soil,  by  a  method  to  be  later 
explained.  A  soil  in  the  best  condition  for  the  respiration  of  roots 
has  the  structure  represented,  under  large  magnification,  in  the 
accompanying  picture  (figure  30).  Soil  is  formed  of  particles 


FIG.  ,30. — A  generalized  drawing  of  a  section,  highly  magnified,  through  a  well-conditioned 
soil  and  a  fragment  of  root.  The  soil  particles  are  dotted,  the  water  is  concentrically- 
lined,  the  air  spaces  are  left  blank;  into  the  soil  project  the  root-hairs  from  the  root 
on  the  left.  (Improved  from  a  picture  in  Sachs'  Lectures.) 

of  rock,  irregular  in  size  and  form.  Around  these  particles  and 
in  the  angles  between  them  is  water,  held  in  the  capillary  state, 
while  bubbles  of  air  exist  in  the  larger  of  the  spaces  among  the  soil 
particles.  When  more  water  is  added,  then  the  air,  being  lighter, 
is  driven  upwards  and  comes  bubbling  out  of  the  ground;  but 
it  returns  again  as  the  surplus  water  drains  or  evaporates  away. 
It  is  from  this  air  in  the  soil  that  roots  take  their  oxygen,  and  if 
the  air  is  kept  out  of  the  soil  by  excess  of  water,  then  the  roots  are 
suffocated  and  die,  precisely  as  air-breathing  animals  do  when  they 


The  Kinds  of  Work  That  Are  Done  by  Plants       87 

are  kept  under  water.  Roots,  in  fact,  drown  as  truly  and  in  ex- 
actly the  same  physiological  way  as  do  animals,  and  with  only 
this  difference,  that  roots  can  stand  immersion  for  hours  or  days, 
while  animals  can  endure  it  only  for  minutes.  This  explains 
the  need  for  drainage  of  wet  soils;  it  is  not  that  these  have  too 
much  water,  but  too  little  air.  It  explains  also  why  the  soil  of 
flower  pots  needs  to  be  carefully  drained,  and  the  cause  of  the 
failure  of  so  many  persons  in  the  care  of  their  house  plants,  which 
most  people  keep  too  constantly  wet.  The  very  best  treatment 
for  most  potted  plants  is  to  give  to  the  soil  an  occasional  soaking, 
and  allow  it  to  dry  out  pretty  well  in  between  times;  the  roots  do 
not  mind  the  absence  of  air  for  some  of  the  time  if  they  can  have  a 
sufficiency  at  other  times.  Moreover  this  method  of  watering  has 
another  great  advantage  over  that  of  adding  a  little  water  more 
frequently,  in  the  far  greater  effectiveness  with  which  it  drives 
out  the  foul  air  and  ensures  a  fresh  supply. 

Another  economic  phase  of  respiration  is  involved  in  the 
popular  belief  that  it  is  unhealthful  to  keep  house  plants  in  sleep- 
ing rooms.  It  will  now  be  plain  to  the  reader  that  this  belief  is 
correct.  But  in  fact  the  danger  is  slight.  The  amount  of  carbon 
dioxide  given  off  in  respiration  by  a  square  meter  of  leaf  is  only 
about  the  three-hundredth  part  of  that  given  off  in  the  same  time 
by  a  person,  and  although  buds  and  roots  respire  more  actively, 
it  is  likely  that  a  whole  window-full  of  plants  does  not  give  off 
one  fiftieth  of  the  amount  that  one  person  does.  Or,  it  has  been 
stated  thus,  that  all  of  the  plants  which  could  be  crowded  into 
the  windows  of  any  ordinary  sleeping  room  give  off  less  carbon 
dioxide  to  the  air  than  would  a  tiny  light  kept  burning  over  night ; 
and  nobody  would  consider  this  quantity  injurious,  especially 
if  the  room  were  ventilated  as  it  should  be.  Indeed,  were  the 
respiration  of  the  plants  in  a  room  not  negligibly  small,  it  would 
obviously  be  unsafe  for  any  person  to  camp  out  in  a  forest  in 
summer! 

We  must  now  come  back  to  the  more  technical  aspects  of  res- 


88  The  Living  Plant 

piration,  and  examine  more  closely  the  chemical  and  physical 
aspects  thereof.  Since  the  plant,  in  this  process,  absorbs  oxygen 
only,  but  releases  carbon  dioxide,  a  question  is  raised  as  to  the 
source  of  the  carbon.  This  must  come,  of  course,  from  some  of 
the  innumerable  carbon-holding  compounds  inside  of  the  plant, 
but,  for  our  present  purpose  it  does  not  much  matter  from  which, 
since  they  all  are  derived  by  transformation  from  the  basal 
grape  sugar  manufactured  in  the  leaves.  This  grape  sugar,  ac- 
cordingly, is  the  ultimate,  even  though  not  the  immediate  source 
of  the  respiratory  carbon.  Therefore  we  can  state  the  end  prod- 
ucts of  respiration  in  this  wise: — 

In  respiration       C6H12O6    and    02    form      C02        and     H20 
grape  sugar  oxygen         carbon  dioxide  water 

This  general  statement  can  be  given  a  definite  chemical  form 
by  making  the  two  sides  sum  up  alike,  which  requires  these  pro- 
portions:— 

CeH12O6  +  6  02  =  6  CO2  +  6  H>0 

Now  although  this  equation  is  rarely  if  ever  actually  realized 
in  any  particular  case,  (respiration  being  never  so  simple,  but  a 
process  highly  complicated  in  its  details),  it  does  represent  the 
facts  as  to  the  ultimate  materials  and  products,  the  two  extremes 
of  the  process;  and  accordingly  we  may  place  it  in  our  series  of 
conventional  constants  as  the  respiratory  equation.  And  its 
relations  to  the  photosynthetic  equation  will  not  escape  the  notice 
of  the  observant  reader.  The  two  are  the  exact  reciprocals  of 
one  another,  which  fact  is  one  of  the  most  consequential  in  all 
nature,  as  will  presently  appear. 

And  now  we  come  to  a  matter  which  I  wish  to  impress,  the 
strongest  I  can,  on  the  mind  of  the  reader.  The  phenomena  we 
have  thus  far  considered,  including  the  one  which  stands  for 
most  people  as  the  very  embodiment  of  the  process,  viz., — the 
remarkable  exchange  of  the  gases, — are  by  no  means  the  ones  of 
greatest  importance  in  respiration,  but  are  secondary  and  in- 
cidental to  the  central  and  crucial  object  of  the  process,  which 


The  Kinds  of  Work  That  Are  Done  by  Plants      89 

is  this, — the  release  of  energy.  This  release  takes  place  in  a  single 
perfectly  definite  way,  namely,  as  the  result  of  the  invariable 
physical  fact  of  Nature  that  at  the  instant  carbon  unites  chemi- 
cally with  oxygen,  it  matters  not  in  what  place  or  under  what 
circumstances,  energy  is  released.  It  is  for  the  release  of  this 
energy  that  the  process  of  respiration  exists;  and  the  plant  no  more 
respires  for  the  purpose  of  absorbing  oxygen  and  releasing  carbon 
dioxide  than  we  kindle  a  fire  in  the  grate  in  order  to  make  oxygen 
rush  into  the  furnace  or  carbon  dioxide  pour  out  of  the  chimney. 
The  object  of  respiration  and  of  building  the  fire  (i.  e.,  of  com- 
bustion), are  one  and  the  same, — namely,  to  secure  that  energy 
which  is  always  released  at  the  moment  of  chemical  union  of 
carbon  with  oxygen.  Respiration  and  combustion  are  strictly 
homologous  terms,  applying  to  phenomena  which  are  also  homol- 
ogous. In  the  combustion  of  coal,  which  is  carbon,  in  a  grate, 
the  energy  is  released  chiefly  as  heat  (with  some  light);  and  by 
causing  that  release  to  occur  underneath  a  suitable  arrangement 
of  boilers,  pistons  and  wheels,  the  energy  can  be  made  to  produce 
motion  and  thus  do  work,  as  every  steam  engine  is  a  visible  wit- 
ness. In  the  explosion  (which  is  merely  a  rapid  combustion),  of 
gasolene  and  oxygen  inside  the  cylinder  of  an  automobile  engine, 
we  have  exactly  the  same  thing  with  a  very  much  simpler  machin- 
ery. In  respiration  within  the  cells  of  an  animal  or  a  plant,  the 
machinery  is  simpler  still,  but  the  principle  remains  the  same; 
the  energy  is  released  at  the  moment  of  oxidation  under  such 
conditions  that  it  acts  on  the  simple  protoplasmic  machinery 
provided  by  the  plant  in  a  way  to  secure  transformation  into 
motion  and  work.  The  source  of  the  energy  of  the  work  done 
by  the  engine  and  plant  is  identically  the  same;  it  is  only  the  in- 
termediate machinery  which  is  different.  The  nature  of  this 
machinery,  it  is  true,  is  not  at  all  understood  in  the  plant,  but  we 
know  that  something  of  the  kind  must  exist.  The  machinery 
must  also  differ  somewhat  for  the  different  kinds  of  work  that 
plants  and  animals  do;  but  in  all  cases  it  is  driven  by  one  and  the 


QO  The  Living  Plant 

same  power,  which  depends  on  the  energy  released  by  the  oxida- 
tion of  carbonaceous  food.  And  it  may  interest  the  reader  hav- 
ing a  turn  for  figures  to  know  that  the  energy  released  by  the 
respiration  of  sugar  is  just  about  half  of  that  released  by  the  com- 
bustion of  an  equal  weight  of  the  best  coal. 

These  matters  though  clear  on  reflection,  are  hard  to  grasp  in  a 
first  presentation;  and  I  suggest  that  we  rest  a  little  by  consider- 
ing an  incidental  matter  of  interest.  In  the  foregoing  paragraph 
I  implied  that  the  energy  of  respiration  is  not  released  as  heat, 
and  thus  differs  from  combustion.  But  the  implication  is  not 
strictly  correct,  as  is  easily  proven.  If  one  takes  two  handfuls 
of  seeds,  soaks  them,  and  starts  them  growing  and  therefore 
respiring,  kills  one  set  by  hot  water,  places  them  both  in  good 
non-conducting  chambers  provided  with  thermometers,  and  leaves 
them  some  hours,  he  will  notice  a  remarkable  result.  The  ther- 
mometer in  the  living  and  respiring  seeds  will  soon  read  several 
degrees  above  that  in  the  others,  which  are  obviously  similar  in 
all  ways  except  that  they  cannot  respire.  And  further  experi- 
ment shows  that  this  release  of  heat  by  these  respiring  seeds  is  rep- 
resentative of  all  respiring  parts,  and  that  the  release  of  heat  is  a 
constant  accompaniment  of  respiration.  Although  usually  small 
in  amount  this  heat  sometimes  becomes  readily  recognizable. 
Thus  the  rapidly-opening  flowers  of  Aroids  (our  Jack-in-the-Pulpit 
and  its  relatives)  often  show  by  the  thermometer  a  temperature 
several  degrees  above  that  of  the  air;  some  alpine  flowers  can  melt 
their  way  up,  by  aid  of  this  heat,  through  the  snow;  grain  germi- 
nating or  fermenting  in  large  masses  becomes  often  noticeably 
warm;  the  warmth  of  hot  beds  derived  from  fermenting  manures 
has  the  same  origin,  though  here  the  respiration  is  that  of  bac- 
teria or  molds;  and  various  cases  of  spontaneous  combustion, 
where  correctly  reported,  must  have  the  same  origin.  It  does  not 
appear  that  this  heat,  in  plants  at  least,  secures  any  physiologi- 
cal advantage  but  is  rather  an  incidental  result  of  the  physical 
forces  at  work,  very  much  as  incandescent  electric  lamps  made 


The  Kinds  of  Work  That  Are  Done  by  Plants      91 

primarily  to  give  only  light  incidentally  give  much  heat  as  well. 
But  it  is  this  very  same  heat  developed  and  kept  in  regulation 
which  is  the  basis  of  the  uniform  warmth  of  the  animal  body. 
A  few  pages  earlier  it  was  shown  that  the  carbon  in  the  carbon 
dioxide  released  in  respiration  comes  from  inside  the  plant.  This 
being  so,  respiration  ought  always  to  entail  a  loss  of  weight  in 


FIG.  31. — Plants  of  Buckwheat  grown  from  the  same  number  and  weight  of  seed  in  light 
and  darkness  respectively.  The  plants  are  in  porous  saucers  supplied  with  water  and 
minerals  from  below. 

respiring  plants  or  animals;  which  in  fact  is  found  by  experiment 
to  be  true.  The  loss  must  be  compensated  by  new  supplies  of 
food,  else  the  phenomena  of  starvation,  including  emaciation, 
ensue.  The  emaciation  of  a  starved  animal,  indeed,  is  due  much 
more  to  the  loss  of  substance  through  respiration  than  through 
the  ordinary  excretions.  In  plants,  however,  it  often  happens 
that  those  which  have  lost  much  weight  by  respiration  without 
opportunity  to  make  it  up  by  photosynthesis,  look  larger  than 


92  The  Living  Plant 

others  which  have  done  the  normal  photosynthetic  work,  the  ex- 
tra bulk  being  nothing  but  water.  Thus,  the  two  sets  of  plants 
in  the  accompanying  picture  (figure  31),  were  started  by  the  water- 
culture  method,  (later  to  be  explained),  from  two  sets  of  seeds  of 
exactly  the  same  weight.  But  one  set  (that  on  the  left)  was  grown 
in  the  light  and  was  able,  therefore,  to  make  up  its  loss  by  photo- 
synthesis, while  the  other  was  grown  in  the  dark  and  could  not. 
Yet  the  latter,  owing  to  the  habit  of  plants  to  spindle  out  greatly 
in  length  in  darkness,  actually  look  larger  than  the  former. 
When,  however,  I  weighed  these  two  sets  after  all  of  the  water 
has  been  dried  out,  leaving  only  dry  substance  behind,  the  smaller 
lighted  plants  weighed  a  good  deal  more  than  the  larger  ones 
from  the  dark.  It  can  always  be  accepted  as  true  that  respiration 
entails  loss  of  weight  through  the  loss  of  carbon  from  the  plant. 

We  can  now  gather  up  the  facts  set  forth  in  the  preceding  pages 
in  another  of  our  generalizations,  or  verities, — the  energy  indis- 
pensable to  the  work  of  plants  is  principally  provided  by  the  oxida- 
tion of  carbonaceous  food,  and  this  is  the  essential  feature  of  respira- 
tion. 

In  the  statement  of  the  foregoing  verity  the  reader  will  notice 
that  I  have  used  the  word  " principally,"  thus  implying  that 
some  other  source  of  energy  is  available.  In  fact,  while  respiration 
supplies  by  far  the  larger  part  of  the  energy  used  by  organisms, 
and  especially  by  animals,  they  do  derive  some  small  part  from 
other  sources,  notably  the  heat  of  the  surroundings.  But  this 
part  of  the  subject  will  all  be  elucidated  later  in  this  book. 

We  are  now  face  to  face  with  a  question  of  a  very  fundamental 
sort, — namely,  what  is  the  source  of  that  energy  which  is  thus 
released  from  food  in  respiration?  For  everybody  knows  that 
energy  is  not  created  upon  the  spot,  but  originates  only  by 
transformation  of  pre-existing  energy.  In  all  science  there  is  no 
principle  better  established,  or  more  important,  than  that  of  the 
conservation  of  energy  and  matter,  which  teaches  that  the  sum 
total  of  both  energy  and  matter  in  nature  is  constant,  and  that 


The  Kinds  of  Work  That  Are  Done  by  Plants      93 

none  of  either  is  ever  created  anew  or  obliterated,  though  they 
may  change  their  forms  multifariously.  Where,  then,  and  in 
what  form  was  the  energy  in  food  before  it  was  released  by  respir- 
ation? The  answer  is  easy,  though  its  comprehension  is  not. 
It  was  where  the  energy  was  in  the  coal  before  it  was  released 
as  heat  in  combustion:  where  the  energy  was  in  the  storage  bat- 
tery before  it  turned  the  wheels  of  the  electric  automobile :  where 
the  energy  was  in  the  coiled  spring  or  the  wound-up  weight  of 
the  clock  before  it  turned  the  wheels  to  move  the  hands:  where 
the  energy  was  in  the  full  millpond  before  it  drove  the  looms  of  the 
water-power  mill :  where  the  energy  was  in  the  gunpowder  before 
it  started  the  flying  bullet.  The  fact  of  the  matter  is  this, — that 
energy  exists  in  Nature  in  two  different  forms,  not  only  in  the 
familiar  active  or  kinetic  form  which  produces  motion  and  does 
work,  but  also  in  a  resting,  latent,  or  potential  form,  when  its 
power  to  produce  motion  is  held  in  suspension.  Whenever,  in 
Nature,  kinetic  energy  is  exerted  to  force  apart  bodies  whose 
attractions,  whether  through  gravitation,  magnetism,  cohesion, 
or  chemical  affinity,  tend  to  bring  them  together,  the  energy  goes 
into  the  potential  form  for  so  long  as  those  bodies  are  kept  apart, 
and  it  becomes  again  manifest  in  kinetic  form  when  the  bodies 
are  allowed  to  re-unite.  All  unsatisfied  attractions  in  Nature  are 
latent  energy.  When  a  small  boy  draws  back  the  powerful  elastic 
of  his  favorite  sling-shot,  he  is  exerting  kinetic  energy  against 
the  tension  of  the  elastic;  while  he  holds  the  elastic  stretched  to 
take  aim,  that  energy  is  latent  as  energy  of  tension;  and  when  he 
lets  go  of  the  string  the  energy  becomes  kinetic  again  as  it  drives 
the  stone  in  delightful  swiftness  of  flight.  So,  kinetic  energy  can 
raise  a  weight,  go  into  the  latent  form  as  energy  of  position  while 
it  is  suspended,  and  come  out  again  in  kinetic  form,  as  it  does 
when  it  turns  the  wheels  of  an  old-fashioned  clock.  Kinetic 
energy  can  charge  a  storage  battery,  become  latent  for  a  time, 
and  come  out  once  more  as  kinetic  energy  driving  an  electric 
automobile.  The  storage  battery,  indeed,  is  typical  of  all  cases 


94  The  Living  Plant 

where  energy  is  potential  in  the  form  of  unsatisfied  chemical 
affinity.  The  electric  current  forces  apart  the  tightly-cohering 
atoms  of  certain  very  stable  chemical  compounds;  but  these  atoms 
nevertheless  retain  all  their  old  attraction  for  one  another,  and 
it  is  in  the  form  of  this  unsatisfied  attraction  that  the  energy 
is  latent;  and  this  energy  is  given  out  again  in  kinetic  form  at 
the  moment  when  the  atoms  are  allowed  once  more  to  unite. 
Now  the  very  same  thing  is  true  of  carbon  dioxide,  which  is  a 
very  stable  substance  of  tightly-cohering  atoms.  To  force  apart 
carbon  dioxide  into  its  constituents  requires  kinetic  energy, 
which  then  remains  in  the  latent  form,  as  energy  of  unsatisfied 
chemical  affinity,  so  long  as  the  carbon  and  oxygen  are  held  apart, 
but  becomes  kinetic  again  when  the  carbon  and  oxygen  are  al- 
lowed to  reunite  to  carbon  dioxide.  Does  the  reader  see  the  ap- 
plication? Surely  he  must.  The  kinetic  energy  of  the  sunlight 
splits  apart  carbon  dioxide  in  the  green  leaf,  the  oxygen  going 
out  to  the  air  and  the  carbon  combining  with  the  elements  of 
water  into  grape  sugar;  so  long  as  this  carbon  and  oxygen  are  kept 
apart,  that  energy  is  latent  in  the  form  of  unsatisfied  chemical 
affinity;  and  when  the  carbon  of  the  sugar  (or  of  any  other  sub- 
stance into  which  the  sugar  is  transformed)  is  allowed  to  unite 
with  the  oxygen  of  the  air,  as  it  is  in  the  process  of  respiration, 
then  kinetic  energy  is  again  given  out  and  can  be  used  for  the  work 
of  the  plant.  Such  is  the  source  of  the  energy  of  respiration, — 
it  is  energy  released  from  the  latent  state  in  food,  where  it  was 
placed  (or  "stored")  by  the  kinetic  energy  of  the  sunlight.  Food, 
therefore,  is  a  storage  battery  charged  by  the  sun,  and  discharged 
by  respiration. 

The  principal  function  of  food  must  now  be  quite  plain.  As  a 
storage  battery  it  has  advantage  over  any  that  man  has  yet 
made  in  the  fact  that  it  can  be  reduced  to  very  small  fragments, 
or  even  to  solution  (by  digestion),  and  thus  transported  to  all 
parts  of  plants  and  throughout  the  bodies  of  animals.  Then,  at 
the  spot  where  work  needs  to  be  done,  just  at  the  right  instant, 


The  Kinds  of  Work  That  Are  Done  by  Plants      95 


under  the  suitable  machinery,  the  carbon  of  the  food  is  allowed 
to  unite  with  oxygen,  and  the  energy  is  released  to  do  the  need- 
ful work.  And  that  is  the  way  in  which  plants  and  animals  ac- 
complish their  work;  and  the  power  to  do  this,  —  to  absorb  stored 
energy,  transfer  it  to  all  of  their  parts,  hold  it  ready  for  use,  and 
release  it  when  needed,  —  is  the  most  distinctive  feature  of  living 
beings. 

The  reason  is  now  evident  also  for  the  reciprocal  character 
of  the  photosynthetic  and  respiratory  equations.  In  photosyn- 
thesis carbon  dioxide  and  water  are  made  into  sugar  and  oxygen 
with  storage  of  energy;  the  sugar  is  transported  by  plants  or  by 
animals  to  places  of  need,  undergoing  chemical  changes  on  the 
way  but  ever  retaining  the  store  of  unsatisfied  carbon;  then  in 
respiration  oxygen  is  allowed  to  come  into  chemical  contact  with 
the  sugar,  and  the  two  are  changed  back  to  carbon  dioxide  and 
water  with  release  of  energy.  It  is  because  substances  exist  which 
thus  permit  of  such  storage  and  transportation  of  energy  that 
organisms  as  we  know  them  are  possible. 

It  may  aid  still  more  to  a  clear  understanding  of  these  two  most 
fundamental  and  important  of  all  physiological  processes  if  we 
set  their  chief  features  in  contrast  in  form  of  a  table;  — 


Photosynthesis 

Occurs  only  in  plants 

Occurs  only  in  chlorophyll  grains 

Occurs  only  in  light 

Manufactures  food 

Increases  weight 

Absorbs  carbon  dioxide 

Releases  oxygen 

Forms  CeH^Oe  from  C02  and  H20 

Stores  energy 


Respiration 

Occurs  equally  in  plants  and  animals 

Occurs  in  all  living  protoplasm 

Occurs  equally  in  light  and  darkness 

Destroys  food 

Lessens  weight 

Releases  carbon  dioxide 

Absorbs  oxygen 

Reduces  C6Hi2O6  to  CO2  and  H20 

Releases  energy 


We  can  now  gather  up  these  latter  facts  in  another  of  our 
verities  thus,  —  the  energy  released  in  respiration  was  previously 
latent  in  the  unsatisfied  affinity  of  the  carbon  in  the  food  for  the 


96  The  Living  Plant 

oxygen  outside,  those  two  elements  having  originally  been  separated 
by  the  kinetic  energy  of  the  sunlight  in  photosynthesis  and  kept 
separate  through  all  the  subsequent  transformations  and  trans- 
portations of  the  food  through  the  bodies  of  plants  and  animals;  the 
original  source  of  respiratory  energy  is  therefore  the  sunlight,  and 
food  is  primarily  a  storage  battery,  charged  'by  the  sun  in  green 
leaves  and  discharged  by  respiration  at  the  places  of  need. 

It  will  doubtless  ere  this  have  occurred  to  some  philosophic 
reader  to  ask  whether  carbon  dioxide  and  water  are  the  sole 
substances  by  which  organisms  can  thus  store  and  transport 
energy,  and  whether,  accordingly,  life  is  dependent  solely  upon 
them.  There  is,  however,  no  chemical  reason  why  organisms 
might  not  use  in  the  same  way  any  other  decomposable  and 
oxidizable  substances,  and  indeed  even  in  our  common  plants  some 
small  quantity  of  energy  is  no  doubt  derived  from  the  oxidation  of 
other  elements,  while  certain  Bacteria  exist  which  can  use  the 
energy  derived  from  the  oxidation  of  sulphur  compounds.  Plants 
probably  use  carbon  in  photosynthesis  and  respiration  chiefly 
because  its  chemical  transformations,  which  are  very  susceptible 
to  temperature,  happen  to  be  easily  under  control  at  the  temper- 
atures now  prevailing  on  the  earth's  surface.  Under  markedly 
higher  or  lower  temperatures  carbon  would  be  unavailable  for  this 
purpose,  but  it  is  conceivable  that  life  might  still  exist  by  the 
similar  use  of  other  substances  whose  combinations  would  be 
under  control  at  those  temperatures.  It  is  only  a  step  farther  to 
assume  that  life  might  even  exist  in  this  way  in  the  flames  of  a 
nebula,  or  the  awful  cold  of  interplanetary  space,  and  hence 
that  its  origin  may  be  contemporaneous  not  only  with  the  origin 
of  the  earth,  but  even  with  the  origin  of  matter  itself.  It  is  not 
at  all  likely  that  life  is  something  which  results  incidentally  from 
the  properties  of  carbon;  it  is  far  more  probable  that  it  is  some- 
thing which  uses  the  properties  of  carbon  as  the  most  convenient 
tools  for  its  own  ends.  This  is  a  phase  of  the  super-vitalism  of 
which  I  have  spoken  in  the  first  chapter. 


The  Kinds  of  Work  That  Are  Done  by  Plants      97 

This  chapter  has  already  attained  to  a  length  so  great  that  I 
wish  it  were  possible  to  end  it  right  here.  But  certain  additional 
matters  are  connected  with  respiration  so  closely,  and  are  be- 
sides in  themselves  so  important,  that  we  must  really  keep  on  to 
include  them,  though  perhaps  the  reader  will  find  it  best  to  defer 
a  reading  thereof  for  another  occasion.  These  matters  are  fer- 
mentation, decay,  and  disease. 

Fermentation  is  a  phenomenon  familiar  to  all,  and  best  known, 
perhaps,  in  the  "  working"  of  preserves,  which  become  "  strong" 
i.  e.  alcoholic,  while  giving  off  tiny 
bubbles  of  gas.  The  most  typical 
kind  of  fermentation  is  that  caused 
by  Yeast.  Yeast,  I  venture  to 
remind  the  reader,  is  a  very  tiny 
non-green  plant  which  lives  as  a 
saprophyte  in  sweet  liquids.  Mag- 
nified to  a  high  degree  by  the  mi- 
croscope it  looks  much  like  our 
picture  (figure  32),  though  whiter.  , 

FIG.  32. — Yeast  plants,  each  a  single  cell 

A   Yeast    plant    is    a    Single    OVOid        which  buds  out  from  a  parent  cell;  very 
„        I.,,        i  .     .     ,  highly  magnified. 

cell  which  buds  out  into  others, 

and  these  into  others,  in  loose  chains  which  fall  easily  apart, — 
and  so  on,  as  long  as  the  food  supply  lasts.  And  that  is  all, 
except  that  when  the  liquid  dries  up,  the  cells  produce  very 
thick-walled  spores  which  float  around  in  the  air  with  the  dust, 
to  start  once  more  when  they  happen  to  fall  into  another  sweet 
liquid.  It  is  by  the  growth  of  these  cells  that  a  sweet  liquid  is 
"fermented"  with  a  formation  of  alcohol  and  carbon  dioxide. 
This  can  be  demonstrated  very  easily  and  clearly  to  the  eye  by 
an  interesting  experiment.  If  one  puts  together  in  a  glass  flask 
a  solution  of  sugar  and  a  cake  of  compressed  (not  dried)  yeast, 
and  stands  it  in  a  warmish  place,  then  within  a  very  few 
minutes  tiny  bubbles  of  gas  begin  to  rise  through  the  liquid, 
producing  a  froth  on  its  surface.  If,  now,  the  stopper  of  the  flask 


98  The  Living  Plant 

be  provided  with  an  outlet  tube  bent  over  to  end  at  the  bottom 
of  a  vessel  of  clear  limewater,  the  gas  will  come  bubbling  up,  and 
will  soon  turn  the  limewater  milky,  thus  proving  its  identity. 
And  when  the  fermentation  is  ended  the  liquid  left  in  the  flask 
has  always  that  " sourish"  smell  distinctive  of  the  presence  of  al- 
cohol, which,  indeed,  can  be  separated  for  testing  by  distilling 
the  liquid.  As  to  its  quantity,  however,  it  is  important  to  know 
that  even  when  all  the  conditions  for  fermentation  are  most 
favorable  and  the  sugar  is  present  in  plenty,  the  Yeast  neverthe- 
less does  not  form  more  than  a  limited  quantity  of  alcohol, — 
(about  ten  per  cent  of  the  liquid  in  round  numbers),  for  then  the 
plant  is  rendered  inactive  and  may  finally  be  killed  by  the  very 
alcohol  which  it  has  produced. 

Such  is  the  process  of  fermentation,  which,  as  everybody  knows, 
is  vastly  important  in  the  arts.  Sometimes  it  is  used  for  the  sake 
of  its  carbon  dioxide  and  sometimes  for  the  sake  of  its  alcohol. 
The  conspicuous  case  of  the  former  is  found  in  the  making  of 
bread,  where  the  carbon  dioxide  released  from  the  growth  of  the 
yeast  cells  throughout  the  mass  of  the  dough,  forms  the  cavities 
by  which  it  is  lightened  and  raised.  When  everything  goes  as  it 
should,  the  alcohol  evaporates  hi  the  baking,  but  sometimes 
it  does  not,  and  then  the  bread  goes  "sour. "  Of  course  other 
methods  of  raising  bread  are  in  use,  notably  by  aid  of  gases  re- 
leased in  the  dough  from  chemical  action  between  the  constit- 
uents of  suitable  "baking  powders,"  or  other  substances,  and 
also  by  use  of  air  blown  into  the  dough;  but  yeast  fermentation 
is  much  the  most  used  of  the  methods.  But  far  more  extensive 
is  the  employment  of  fermentation  for  the  making  of  the  various 
kinds  of  alcoholic  liquids.  When  the  sweet  juice  of  the  grape  is 
allowed  to  ferment  (by  action  of  yeast  blown  as  spores  through 
the  air  to  the  fruits),  the  carbon  dioxide  escapes  to  the  air,  and 
the  remaining  admixture  of  alcohol,  water,  and  flavors  we  call 
wine.  When  the  sweet  pulp  of  the  germinating  grains  of  barley 
is  allowed  to  ferment  (by  Yeast  which  is  added  for  the  purpose), 


The  Kinds  of  Work  That  Are  Done  by  Hants      99 

we  give  the  name  beer,  " lager  beer,"  to  the  liquid  resulting. 
And  innumerable  other  sweet  juices  and  saps  are  fermentable, 
with  resulting  formation  of  alcoholic  beverages,  which  are  so 
many  and  diverse  in  kind  that  most  nations  have  each  some 
favorite  one  of  its  own,  the  differences  between  them  being  due 
in  the  main  to  various  flavoring  materials  originally  present  with 
the  sugar.  None  of  these  fermented  liquids,  however,  are  ever 
stronger  in  alcohol  than  the  ten  per  cent,  or  thereabouts,  which 
the  Yeast  can  yield  before  it  is  killed.  The  stronger  liquors  are 
obtained  by  an  additional  and  very  different  kind  of  operation,  de- 
pending on  the  fact  that  alcohol  boils  at  a  much  lower  temperature 
than  water  (78°C,  or  172°F  as  compared  with  100°C  or  212°F). 
For  this  reason  a  fermented  liquid,  if  heated  above  78°  but 
under  100°,  gives  off  its  alcohol  (though  also  with  some  water) 
as  vapor,  which  can  be  conducted  away,  cooled  and  collected 
as  a  strongly  alcoholic  liquid.  The  process  is  called  distillation, 
and  in  this  way  are  made  the  stronger  alcoholic  drinks, — brandy, 
whisky,  rum,  gin,  and  all  the  remainder  of  this  precious  rogue's 
gallery, — their  peculiar  flavors  and  colors  being  due  to  particular 
substances,  sometimes  naturally  present  and  sometimes  purposely 
added,  in  the  juices  from  which  the  alcohol  is  fermented.  It  is  by 
repeated  distillation  of  the  fermented  juice  of  germinating  corn 
that  the  strong  alcohol  of  commerce  is  made,  and  this  when  mixed 
with  a  little  of  the  poisonous  wood  alcohol  to  make  it  undrinkable 
becomes  the  "denatured  alcohol"  of  the  household  and  the  chaf- 
ing dish. 

We  turn  now  to  the  chemistry  of  fermentation,  which  is  simple. 
It  is  grape  sugar  which  is  fermented,  for  other  sugars  or  starches 
are  first  changed  to  that  form  or  its  equivalent.  Therefore  we 
have  this  expression, 

In  fermentation         C6H1206     forms       C02          and    C2H6O 

grape  sugar  carbon  dioxide  alcohol 

This  statement  can  be  given  an  exact  chemical  form  in  this 

way,— 


ioo  The  Living  Plant 

C6H1206  =  2  C02  +  2  C2H60 

And  this  equation  expresses  exactly  the  known  facts  of  the 
process. 

What  now  is  the  meaning  of  fermentation,  and  why  does  the 
Yeast  do  it?  Nowhere  in  Nature,  so  far  as  I  can  find,  excepting 
in  the  case  of  humanity,  is  there  even  the  least  evidence  that  any 
kind  of  organism  ever  does  anything  whatever  for  the  sake  of 
service  to  any  other  kind.  We  should  not  expect  to  find,  accord- 
ingly, that  the  Yeast  makes  the  carbon  dioxide  and  alcohol  for 
any  disinterested  or  philanthropic  purposes, — not  for  providing 
thrifty  housewives  with  light  bread  or  their  shiftless  husbands 
with  strong  drink, — and  we  turn  to  seek  some  desirable  object  of 
its  own  to  which  the  use  by  mankind  is  purely  incidental.  But 
of  course,  the  reader  has  inferred  the  explanation  before  this, — 
fermentation  is  simply  the  Yeast's  respiration,  the  source  of  its 
power  for  growth  and  other  work  that  it  does.  And  the  explana- 
tion of  so  peculiar  a  form  of  respiration  is  well  known.  Living  im- 
mersed in  a  liquid,  the  Yeast  cannot  obtain  respiratory  oxygen 
from  the  air,  and  must  take  it  from  some  other  source.  Only  one 
source  is  available.  Locked  up  in  the  molecule  of  sugar  is  some 
oxygen  brought  into  it  with  the  hydrogen,  which  holds  it  away 
from  the  carbon,  as  the  formula  C6H1206  suggests.  But  the  Yeast 
plant,  absorbing  the  sugar  into  its  body,  shatters  the  molecules 
(by  means  of  a  peculiar  agency  called  an  enzyme  soon  to  be 
described),  and  allows  the  carbon  and  oxygen  in  the  fragments 
to  unite  with  one  another;  this  produces  the  usual  result, — a 
copious  release  of  energy  which  the  Yeast  at  once  utilizes  for  its 
growth,  while  of  course  the  resulting  carbon  dioxide  is  thrown  off 
into  the  liquid.  This  is  the  object,  or  meaning,  of  fermentation; — 
to  secure  a  union  of  carbon  and  oxygen  for  the  sake  of  the  energy 
which  is  always  thus  released.  As  to  the  alcohol,  that  is  simply 
the  remains  of  the  shattered  molecule;  it  is  a  chemical  fact  that 
the  number  of  atoms  of  carbon,  hydrogen  and  oxygen  which  hap- 
pen to  be  left  after  the  carbon  dioxide  is  formed,  fall  naturally 


The  Kinds  of  Work  That  Are  Done  by  Plants     101 

into  alcohol,  and  the  Yeast  plant  cannot  help  it.  That  is  why  the 
Yeast  produces  the  poisonous  alcohol,  despite  the  suicidal  char- 
acter of  the  proceeding.  The  Yeast,  however,  can  respire  in  no 
other  way,  and  with  commendable  philosophy,  prefers  a  short 
life,  even  at  the  risk  of  an  alcoholic  grave,  to  no  life  at  all.  Yet 
in  fact  the  case  is  not  really  so  bad,  for  the  alcohol  is  very  volatile, 
and  in  Nature  commonly  evaporates  as  rapidly  as  formed;  and 
even  when  not,  the  drying  up  of  the  liquid  and  spore-formation 
allow  the  yeast  to  escape  and  renew  its  activity  at  another  time 
and  place.  If  the  Yeast  plant  had  nothing  to  do  but  respire, 
the  sugar  would  all  be  converted  to  carbon  dioxide  and  alcohol, 
which  are  probably  the  sole  products  of  its  respiration.  But  the 
Yeast  must  also  make  new  substance,  protoplasm  and  walls, 
for  which  purpose  it  uses  some  of  the  sugar  in  a  different  wray, 
along  with  other  substances,  and  thereby  develops  incidentally 
a  small  percentage  of  by-products, — glycerin,  acids,  etc.,  the  pur- 
suit and  capture  of  which  affords  a  fine  joy  to  the  special  student 
of  chemistry,  especially  if  some  student  of  biology  has  previously 
told  him  that  carbon  dioxide  and  water  are  the  "products  of  fer- 
mentation." 

Alcoholic  fermentation  caused  by  Yeast  is  the  most  typical 
and  familiar  kind,  but  other  sorts  occur,  caused  by  germs  (Bac- 
teria), or  Molds.  Thus  the  souring  of  milk,  the  rancification 
of  butter,  the  genesis  of  vinegar,  and  even  the  development  of 
distinctive  flavors  in  ripening  cheese,  are  products  of  fermenta- 
tions, caused  in  their  respiration  by  various  organisms.  As  these 
cases  illustrate,  the  secondary  products  need  by  no  means  con- 
sist only  of  alcohol,  but  can  include  substances  of  the  most  diverse 
chemical  natures.  All  that  is  requisite  is  that  carbon  and  oxygen 
shall  be  allowed  to  unite;  the  matter  of  the  particular  compounds 
is  secondary. 

If  any  doubt  could  exist  that  fermentation  is  simply  the  respir- 
ation of  the  Yeast  plant,  it  would  vanish  before  the  remarkable 
fact  that  an  exactly  intermediate  step  is  known  between  the 


102  The  Living  Plant 

respiration  of  the  higher  plants  and  typical  fermentation.  Ideally, 
in  the  respiration  of  the  higher  plants,  the  oxygen  absorbed  and 
carbon  dioxide  released  are  equal  in  volume,  but  often  they  are 
not.  Thus,  some  kinds  of  seeds,  like  Peas,  if  shut  away  from 
oxygen,  can  release  plenty  of  carbon  dioxide  without  absorbing 
any  oxygen  at  all;  and  analysis  of  the  seeds  then  shows  the  pres- 
ence of  alcohol.  In  other  words,  these  Peas,  like  the  Yeast  plant, 
can  cause  fermentation  (though  in  limited  degree)  of  some  of 
their  own  substance;  and  there  is  no  doubt  that  it  represents  the 
form  of  respiration  to  which  the  seeds  resort  when  no  oxygen 
from  the  air  is  available.  This  form  of  fermentation  is  called 
in  the  Peas,  and  the  other  plants  which  make  use  of  it,  anaerobic, 
or  intramolecular,  respiration. 

There  remain  two  other  forms  of  fermentation  so  important 
as  to  require  a  separate  treatment.  One  is  decay,  or  putrefaction, 
which  is  really  the  fermentation  of  dead  plant  and  animal  sub- 
stances by  Bacteria,  or  germs.  Bacteria  are  plants  even  smaller 
and  simpler  than  Yeasts.  The  products  of  their  respiration  and 
growth  are  most  diverse,  including  not  only  carbon  dioxide  and 
water  but  various  other  gases,  some  of  which  possess  those  very 
vile  odors  distinctive  of  rotting  organic  matter.  When  the  de- 
caying substances  are  complex,  e.  g.,  flesh  or  other  proteins,  certain 
Bacteria  ferment  them  to  simpler  sorts,  other  kinds  to  simpler 
still,  and  so  on,  until  they  are  finally  reduced,  as  in  ordinary  respir- 
ation, to  carbon  dioxide  and  water,  and  such  other  elemental 
substances,  (e.  g.,  nitrogen)  as  may  also  have  entered  into  their 
composition.  All  decay  is  simply  a  form  of  fermentation,  that  is 
respiration,  by  Bacteria,  or,  in  some  cases,  by  simple  Molds. 

Another  phase  of  the  same  phenomenon  is  involved  in  those 
deadly  diseases  which  are  caused  by  Bacteria, — Asiatic  Cholera, 
Tuberculosis,  Diphtheria,  Typhoid,  Lockjaw,  and  a  number  of 
others.  It  is  a  popular  belief  that  Bacteria  produce  their  effect 
in  disease  by  destroying  the  tissues,  or,  as  a  plain-spoken  student 
of  mine  once  expressed  it,  they  "chew  you  all  up  inside."  That 


The  Kinds  of  Work  That  Are  Done  by  Plants     103 

belief  is  far  from  the  truth,  for  what  happens  is  this.  The  Bac- 
teria, in  order  to  obtain  energy  and  material  for  their  own  pro- 
cesses, act  on  the  tissues  or  the  blood  in  just  the  same  way  that 
Yeast  acts  on  the  sugar,  likewise  forming  incidentally  in  the  act 
various  accessory  substances.  Now  some  of  these  substances, 
bearing  much  the  same  relation  to  the  Bacteria  that  alcohol 
does  to  the  Yeast,  are  those  alkaloids  or  ptomaines  which  happen 
to  be  violently  poisonous  to  man,  and  it  is  these  poisons,  and  not 
the  Bacteria  directly,  which  are  the  cause  of  his  death.  At  least 
they  are  the  cause  of  his  death  if  they  are  formed  more  rapidly 
than  his  system  can  antagonize  them,  for  the  body  has  a  wonder- 
ful power  of  forming  antagonistic  chemical  substances,  or  anti- 
bodies, which  neutralize  these  poisons, — which  antibodies,  by  the 
way,  can  be  made  to  form  in  the  body,  or  even  can  be  injected 
as  antitoxins,  ensuring  immunity  against  some  diseases.  These 
deadly  diseases  are  therefore  an  incidental  result  of  the  respiration 
and  growth  of  Bacteria  which  are  leading  their  own  lives  in  their 
own  way,  as  oblivious  to  any  harm  they  may  do  as  is  the  Yeast 
to  the  benefit  it  confers. 

It  is  not  only  true  that  fermentation,  decay,  and  some  disease, 
are  caused  by  the  activity  of  Yeasts,  Molds,  and  Bacteria,  but 
the  converse  is  equally  well-known, — that  those  processes  occur 
through  no  other  agency  and  can  be  prevented  entirely  by  killing 
these  organisms.  This  can  be  done  by  heat,  poisons,  certain 
strong  solutions,  or  even,  in  some  cases,  bright  light;  and  such  is 
the  basis  of  the  various  sterilizing  and  antiseptic  processes  so 
familiar  in  the  household,  the  arts,  and  in  medicine. 

We  can  now  express  these  later  facts  in  another  of  our  verities 
as  follows; — all  fermentation  and  decay,  and  some  phases  of  dis- 
ease, are  forms  of  the  respiration  of  simple  organisms  which  thereby 
destroy  organic  matter  by  reduction  back  to  the  carbon  dioxide,  water, 
and  other  elements,  from  which  it  was  originally  built  up. 

It  is  thus  evident  that  all  of  the  carbon  dioxide  and  water 
built  into  plant  substance  by  photosynthesis,  are  ultimately  re- 


104  The  Living  Plant 

leased  again  by  respiration  or  decay.  A  quantity,  rather  small, 
of  the  earth's  supply  of  carbon  dioxide  and  water  is  therefore 
always  locked  up  in  plant  and  animal  substance;  but  though  the 
quantity  is  approximately  constant  the  precise  molecules  are 
constantly  changing,  and  with  the  changes  go  those  transforma- 
tions of  energy  which  are  the  principal  manifestation  of  life.  And 
if  the  question  be  asked,  why  are  not  more  of  the  carbon  dioxide 
and  water  of  nature  locked  up  in  plant  and  animal  substance, 
that  is,  why  are  there  not  more  and  larger  plants  and  animals  on 
earth,  I  think  the  answer  is  easy.  There  do  already  exist  upon  the 
earth  all  of  the  plants  and  animals,  and  as  big  ones,  as  the  physical 
conditions  permit.  As  to  plants,  every  spot  on  the  earth  that 
can  maintain  plant  life  at  all  is  bearing  all  the  plants  it  can  sup- 
port, and  these  plants  are  just  as  big  as  the  physical  conditions 
permit  them  to  grow.  As  to  animals,  they  are  dependent  upon 
plants  for  their  food,  and  it  is  evident  that  there  is  available  for 
their  use  only  the  surplus  of  food  produced  by  plants  over  that 
which  these  need  for  themselves, — and  animals  are  just  as  abun- 
dant and  big  as  that  surplus  can  support. 

Thus,  these  apparently  very  complicated  processes  of  photo- 
synthesis and  respiration,  like  many  another  and  probably  like  all 
of  the  physiological  processes  in  plants  and  in  animals,  can  be 
reduced  to  a  basis  of  pure  physics  and  chemistry.  And  we  shall 
learn  later,  in  our  chapters  on  Irritability  and  on  Growth,  that 
we  have  a  good  explanation  of  the  orderly  sequence  and  regular 
connection  of  the  processes  in  their  linking  up  together  through 
their  interactions  as  stimuli.  Is  there  then,  nothing  in  the  plant 
except  the  interactions  of  chemistry  and  physics?  Let  the  remain- 
ing pages  of  this  book  give  their  testimony  before  we  attempt  the 
answer. 


CHAPTER  V 

THE  VARIOUS  SUBSTANCES  MADE  BY  PLANTS,  AND  THE 
USES  THEREOF  TO  THEM  AND  TO  US 

Metabolism 

N  chapter  two  of  this  book  it  was  shown  that  plants 
manufacture  grape  sugar  in  their  lighted  green  leaves; 
and  I  said  it  would  later  be  proven  that  this  sugar  rep- 
resents a  basal  food  substance  out  of  which,  with  sundry 
minor  additions,  plants  build  all  of  their  other  materials.  The 
time  has  now  come  for  this  demonstration,  to  which,  as  a  sub- 
ject possessing  perhaps  more  importance  than  interest,  I  bespeak 
the  reader's  somewhat  spartan  attention.  Since  all  of  the  sub- 
stances constructed  by  plants  have  a  meaning  in  their  vital 
economy,  I  might  also  have  entitled  this  chapter  "on  the  various 
uses  that  plants  make  of  their  food,"  in  which  case  I  should 
have  to  commence  with  a  review  of  respiration,  for  that  is  the 
most  important  of  the  uses  of  food.  The  others  here  follow  in 
an  order  determined  chiefly  by  the  chemical  nature  of  the  sub- 
stances concerned. 

The  number  of  substances  constructed  by  plants  is  verily 
legion,  for  the  vast  variety  of  foods  and  fabrics,  drugs  and  dyes, 
and  other  materials  yielded  by  them  to  us  is  only  a  small  pro- 
portion of  those  which  they  actually  make.  Fortunately,  how- 
ever, for  our  limited  comprehensions,  those  which  are  really 
important  are  few,  and  moreover,  they  fall  into  somewhat  defi- 
nite classes.  Since  the  subject  is  new  to  most  persons,  I  will  give 
these  classes  in  synopsis  as  a  kind  of  table  of  contents  to  this 
chapter.  They  are  these: — 

105 


io6  The  Living  Plant 

Class  I.  The  BASAL  FOOD,  or  PHOTOSYNTHETIC  SUGAR;  the  substance  first 
formed  in  lighted  green  leaves;  composition  CeH^Oc. 

Class  II.  The  FOODS,  active  and  reserve,  and  the  SKELETON;  chemically 
called  CARBOHYDRATES,  with  a  composition  identical  with  or 
readily  transformable  from  that  of  the  photosynthate,  viz., 
CeHiaOe,  or  Ci2H22Oii,  or  (C6Hio05)n. 

Class  III.  The  SECRETIONS;  various  non-nitrogenous  substances,  mostly  of 
special  ecological  functions,  DERIVATIVES  OF  CARBOHYDRATES 
and  containing  the  same  elements,  but  in  markedly  different 
proportions,  and  hence  collectively  expressible  only  in  the  form 
CnHnOn. 

Class  IV.  The  NITROGEN-ASSIMILATES,  chemically  called  AMIDES;  inconspic- 
uous but  important  substances  containing  the  elements  of  the 
photosynthate  with  the  addition  of  nitrogen,  and  forming  the 
transition  from  Class  I  to  Class  VI;  collectively  expressible 
only  as  CnHnOnNn. 

Class  V.  The  PRINCIPAL  POISONS,  chemically  called  ALKALOIDS;  containing 
(as  a  rule)  the  elements  of  the  Amides  but  in  different  pro- 
portions, substances  of  uncertain  meaning,  and  collectively 
expressible  as  CnHn  (On)  Nn. 

Class  VI.  The  FLESH-FORMERS,  chemically  called  PROTEINS,  contributing 
to  the  formation  of  protoplasm  and  consisting  of  the  elements 
of  the  Amides  with  the  addition  of  sulphur  and  phosphorus, 
and  collectively  expressible  only  as  CnHnOnNnSn  (Pn)« 

Class  VII.  The  REGULATORS  OF  METABOLISM,  called  ENZYMES,  substances 
of  unknown  composition,  but  supposed  to  be  proteins,  possess- 
ing remarkable  properties  of  causing  chemical  transformations 
in  other  substances. 

Class  VIII.  LIVING  PROTOPLASM. 


Class  I.     The  Basal  Food,  or  Photosynthetic  Sugar 

This  substance  needs  no  introduction  to  the  reader  of  the  earlier 
parts  of  this  book;  but  for  others  it  may  be  characterized  as  a 
sugar  made  abundantly  in  the  lighted  green  leaves  of  plants  from 
carbon  dioxide  and  water,  and  forming  the  foundation  of  all 
organic  substances.  It  belongs  in  a  class  by  itself  only  because 
of  its  unique  mode  of  formation  and  function,  for  chemically 
it  belongs  in  the  second  class,  being  nothing  other  than  a  mixture 
of  the  grape  and  fruit  sugars  next  to  be  described. 


The  Various  Substances  Made  by  Plants  107 

Class  II.  The  Food  and  Skeletal  Substances,  or  Carbohydrates 
Grape  Sugar.  This  substance  is  formed  abundantly  in  green 
leaves  as  the  photosynthate,  and  is  common  in  nearly  all  parts 
of  all  plants.  It  is,  however,  much  less  known  than  its  import- 
ance would  imply,  because  it  has  no  prominent  economic  uses, 
and  exists  in  the  plant  only  in  solution  in  the  sap  of  the  cells, 
which  therefore  display  through  its  presence  no  more  striking 
appearance  than  that  represented  in  the  accompanying  example 
(figure  33).  However,  it  sometimes  ac- 
cumulates considerably  in  fruits,  which 
it  helps  to  make  nutritious  and  attract- 
ive to  animals  in  connection  with  dis- 
semination, a  subject  to  be  later  dis- 
cussed in  a  special  chapter  devoted  to 
that  subject;  and  in  grapes,  especially, 
it  is  so  plenty  that  it  crystallizes  out 
when  they  are  dried,  forming  the  soft 
sugar  abundant  on  some  kinds  of  raisins.  FIG.  33!— Appearance  in  opti- 
Its  many  and  easy  transformations  into  %  T^^S^SS^ 
other  substances  will  be  traced  in  the  stored  in  the  saP- 

Inside  the  wall  is  a  lining  of  liv- 

folloWing     pages.          It     has,     however,     a        ing  protoplasm  which  encloses 
j  .    .  ,        .        .~  ,  the  large  sap  cavity  wherein 

second  origin  and  significance  in  the  is  water  containing  the  dis- 
plant,  for  it  is  that  into  which  many  other  solved  sugar" 
substances  are  converted  in  digestion,  as  we  shall  presently  learn, 
and  is  the  commonest  form  in  which  substances  are  translocated 
through  the  plant.  It  is  white  in  mass,  looks  amorphous  and  not 
crystalline  to  the  eye,  is  sweet  to  the  taste,  though  much  less 
sweet  than  cane  sugar,  and  is  the  easiest  of  all  sugars  for  Yeast 
to  ferment.  It  is  interesting  to  know  that  it  has  been  made 
artificially  in  the  chemical  laboratory.  Chemically  its  correct 
name  is  dextrose,  though  often  also  called  glucose,  and  its  formula 
is  C6H1206. 

Fruit  Sugar.  This  substance  is  extremely  like  grape  sugar,  with 
which  until  lately  it  was  more  or  less  confounded,  and  with  which 


io8  The  Living  Plant 

it  occurs  in  the  various  roles  above  mentioned  for  grape  sugar. 
It  is  sweeter  than  grape  sugar  but  ferments  less  easily.  Chem- 
ically it  is  called  fructose,  and  has  the  formula  C6H1206,  differing 
from  grape  sugar  not  in  the  kind  or  number  of  atoms  entering  into 
its  composition,  but  in  the  arrangement  of  these  within  the  mole- 
cule, as  best  demonstrated  by  physical  tests  with  polarized  light. 

Cane  Siigar.  This  substance  is  perfectly  familiar  to  everybody, 
for  it  is  the  granulated  sugar  of  the  table.  It  is  widely  spread 
through  plants  dissolved  in  the  sap,  and  accumulates  in  some  kinds 
so  abundantly  as  to  form  a  reserve  supply  of  food  for  them,  and 
a  store  upon  which  animals,  inclusive  of  man,  are  accustomed  to 
draw  for  their  needs.  This  accumulation  occurs  conspicuously  in 
the  Sugar  Cane  and  the  Sugar  Beet  (both  of  which  plants  have 
had  then*  percentage  of  sugar  immensely  increased  by  cultivation), 
hi  the  Maple  tree,  and  in  a  few  other  less  conspicuous  plants, 
while  it  is  common  as  well  in  ripening  fruits.  Chemically  cane 
sugar  is  called  sucrose,  and  has  the  formula  C12H22On.  It  is 
built  up  by  living  protoplasm  from  photosynthetic  sugar  through 
this  simple  step,  2  C6H1206-H20  (water)  =C12H22On;  and  it  falls 
back  by  a  reverse  process  to  a  molecule  of  grape  sugar  and  one  of 
fruit  sugar.  This  latter. step  actually  occurs  in  the  ripening  of 
fruits,  in  cooking,  and  in  digestion;  and  it  is,  therefore,  as  grape 
sugar  or  fruit  sugar  that  cane  sugar  is  finally  incorporated  into 
both  the  plant  and  the  animal  body. 

In  addition  to  these  sugars,  there  are  others  of  rarer  sort  de- 
scribed in  the  technical  books, — all  closely  related  and  more  or 
less  intertransformable  into  those  we  have  mentioned.  Such  are, 
for  example,  maltose,  mannose,  galactose,  arabinose,  xylose, 
fucose.  I  am  very  well  aware  that  these  names  will  have  no  great 
attraction  for  the  reader,  but  I  take  somewhat  the  same  satis- 
faction hi  their  recital  that  Homer  derived  from  the  roll  of  his 
heroes,  whom  also  he  mentions  but  once. 

Starch.  This  substance  is  perfectly  familiar  to  everyone  as 
common  laundry  starch,  and  especially  as  flour,  which  is  mostly 


The  Various  Substances  Made  by  Plants  109 

composed  of  it.  Occurring  as  a  rule  in  tiny  white  grains  scattered 
widely  through  all  kinds  of  tissues,  it  collects  in  some  organs, 
which  swell  very  greatly  for  its  reception.  Such  is  the  Potato, 
which  is  simply  a  starch-storing  underground  stem:  the  Sweet 
Potato,  a  starch-storing  root :  bulbs,  which  are  masses  of  starch- 
filled  leaves:  and  most  seeds,  including  all  of  the  grains,  which 
contain  copious  starch  either  inside  or  around  the  embryo.  In 
all  of  these  cases,  starch  presents  a  characteristic  homogeneous 
firm  whitish  appearance,  contrasting  markedly  with  the  soft 
translucent  aspect  of  structures  in  which  the  food  is  stored  up  as 
sugar,  e.  g.,  the  Sugar  Beet,  Sugar  Corn.  It  happens,  however, 
that  its  presence  can  be  detected  in  a  very  conclusive  way,  namely 
by  the  deep  blue  color  it  assumes  when  touched  by  a  solution  of 
iodine,  as  the  reader  already  has  learned,  and  as  he  can  easily 
prove  for  himself  by  applying  a  little  of  the  tincture  of  iodine  to  a 
lump  of  starch  from  the  laundry  box,  or  to  a  disused  cuff,  or  to 
water  in  which  some  starch  has  been  scraped, — and  heated  until 
it  forms  a  fine  paste.  The  test  is  one  of  the  most  satisfactory 
and  important  in  all  organic  chemistry,  and  so  delicate  that,  by 
its  use  with  the  aid  of  the  microscope,  one  can  detect  even  the 
minutest  quantities  of  starch  in  the  tissues  of  a  plant,  where  it  is 
sometimes  distributed  with  a  curious  and  beautiful  geometrical 
exactness.  It  is  necessary  to  warn  the  experimenter,  that  in 
living  tissues,  however,  the  test  often  works  rather  badly,  because 
iodine  penetrates  active  protoplasm  very  slowly. 

Starch,  when  it  accumulates  in  the  plant,  serves  as  a  store  of 
reserve  food  upon  which  the  plant  can  draw  when  it  starts  new 
growth;  and  starch  is  by  far  the  most  common  and  abundant  of 
plant  foods.  Moreover,  it  serves  equally  well  as  a  food  for  an- 
imals, which,  accordingly,  rob  the  plants;  and  these  are  there- 
fore obliged  as  a  whole  to  make  a  huge  surplus  in  order  to  keep 
any  at  all  for  themselves.  The  importance  of  starch  as  food  for 
man  is  evident  when  one  recalls  that  Wheat,  Corn,  Rice,  Barley, 
Rye, — grains,  which  constitute  the  principal  food  of  the  great 


no  The  Living  Plant 

majority  of  the  human  race, — are  composed  almost  wholly  of 
starch. 

Chemically,  starch  has  the  formula  (C6H1005)n.  It  is  formed 
apparently  thus, — from  dextrose,  C6H12O6,  water,  H20,  is  with- 
drawn, leaving  C6H1005;  this  substance  does  not  occur  in  this 
form  in  the  plant,  but  the  molecules  immediately  aggregate  them- 
selves (chemically,  polymerize),  to  a  considerable  but  unknown 
number,  expressed  by  the  letter  n,  into  compound  molecules. 
Starch  is  made  up  in  this  way  from  dextrose,  and  it  is  of  interest 
to  note  that  a  corresponding  substance  made  from  fructose  occurs 
as  a  reserve  food  dissolved  in  the  sap  of  the  swollen  roots  of  some 
Composite  plants,  where  it  is  called  inub'n.  The  formation  of 
starch  has  never  been  effected  artificially  outside  of  plants,  and 
in  them  it  takes  place  only  inside  of  those  living  protoplasmic 
bodies  called  plastids,  which  include  chlorophyll  grains  and  which 
are  to  be  described  more  fully  in  the  next  chapter.  The  re- 
conversion of  starch  to  dextrose  is  effected  through  the  action  of 
diastase, — one  of  those  remarkable  chemical  agents  called  en- 
zymes, which  we  are  presently  to  study;  and  this  is  exactly  what 
happens  in  the  digestion  of  starch  in  both  the  plant  and  animal 
body.  Indeed,  this  digestion  can  be  carried  on  experimentally 
and  very  easily  in  a  test-tube  by  action  of  diastase  bought  from 
any  chemical  supply  company,  the  disappearance  of  the  starch 
being  proven  by  use  of  the  iodine. 

A  fact  of  another  kind  about  starch  should  be  noticed  at  this 
place.  Even  to  the  unaided  eye  it  looks  granular  in  texture,  while 
the  microscope  shows  that  it  really  is  composed  of  definite  grains, 
which,  moreover,  display  a  remarkable  structure.  If  a  section 
be  cut  from  the  interior  of  a  potato,  for  instance,  and  magnified, 
the  cells  are  found  to  present  an  aspect  well  shown  in  the  typical 
example  here  pictured,  (figure  34).  Within  each  cell  are  numerous 
solid  grains,  various  in  details  of  their  shapes,  but  all  possessing 
in  common  a  focal  spot  near  the  smaller  end,  around  which  are 
excentrically-arranged  layers  (figures  34  and  35).  Starches  from 


The  Various  Substances  Made  by  Plants  in 

other  plants  are  of  different  aspect,  as  our  plate  so  clearly  illus- 
trates (figure  35) ;  but  each  kind  exhibits  characteristics  peculiar 
to  itself,  and  in  general  it  is  true  that  no  two  species  of  plants 
have  grains  exactly  alike,  while  each 
species  has  a  kind  distinctive  of  itself. 
This  fact  has  a  practical  value,  because 
experts  with  the  microscope  can  thus 
learn  to  recognize  the  starches  of  dif- 
ferent plants  at  sight,  and  by  this  means 
can  detect  adulterations  in  starchy  foods 
or  drugs.  Biologically,  also,  this  indi- 
viduality of  the  starches  is  of  very  great 
interest,  for  it  gives  us  a  clear  case  in  FIG.  34.— A  ceil,  highly  magni- 
which  a  well-developed  specific  character  fnegd'  ^*5&££i 
exists  without  any  regard  to  utility;  for  starch  «rains  embedded  in 

.  living  protoplasm. 

even  the  most  radical  adaptationist  would 

hardly  consider  the  forms  of  the  deeply-buried  and  invisible 
starch  grains  as  useful  in  adapting  the  species  to  its  environment. 
And  if  an  internal  specific  character  can  be  useless,  what  need  to 
try  to  explain  every  external  specific  character  as  necessarily 
useful?  I  am  very  well  aware  that  this  little  digression  will  seem 
without  point  to  most  of  my  readers,  but  I  pray  them  to  have 
patience  a  little,  for  I  have  a  good  object.  I  am  calling  their 
attention  when  I  can  to  certain  data  which  will  later  be  useful 
when  we  come  to  consider  the  subject  of  evolution. 

Cellulose.  This  substance  is  vastly  abundant  and  prominent 
in  plants,  for  it  is  the  material  out  of  which  they  construct  the 
walls  of  their  cells  and  therefore  their  entire  firm  skeletons. 
The  reader  can  obtain  a  good  idea  of  pure  cellulose  by  recalling 
the  fibers  of  cotton,  the  pith  of  woody  stems,  or  some  of  the  pur- 
est unstarched  paper,  such  as  the  filter-paper  of  the  laboratories, — 
all  of  which  exhibit  the  distinctive  cellulosian  qualities  of  tough- 
ness, elasticity  and  transparency.  In  some  plants  also,  it  is 
stored  up  as  a  reserve  food  in  the  seed,  when  it  appears  as  an  im- 


112 


The  Living  Plant 


mense  thickening  of  the  cell- wall  (figure  36).  A  conspicuous 
case  is  the  Ivory  Palm,  which  has  seeds  so  hard  as  to  constitute 
a  substitute  for  ivory  in  the  making  of  buttons  and  other  bijou- 
terie, while  the  seed  of  the  Date  owes  likewise  its  stony  hardness 
to  the  same  material.  Though  so  hard,  this  cellulose  is  easily 
digested  to  sugars  by  the  action  of  suitable  enzymes,  and  the  pro- 


FIG.  35. — Typical  grains  of  a  dozen  different  kinds  of  starches,  highly  magnified.    The 
kinds,  in  order  of  arrangement  in  this  picture  are; — 

Potato  Maranta  Pea  Hyacinth 

Wheat  Oats  Sago  Smilax 

Canna  Corn  Bean  Oxalis 

cess  is  applied  commercially  to  ordinary  wood  in  the  manufacture 
of  wood  alcohol.  Naturally,  the  very  cells  which  make  cellulose 
have  the  power  to  digest  it  away  once  more  where  needful;  and 
this  is  why  cell-walls,  even  when  well  grown,  can  become  perfo- 
rated, absorbed,  split,  or  even  re-adjusted  in  such  a  way  that  they 
seem  to  have  slid  upon  one  another. 


The  Various  Substances  Made  by  Plants  113 

Chemically,  cellulose  is  related  to  grape  sugar  and  formed  there- 
from in  much  the  same  way  that  starch  is,  its  formula  being  the 
same  as  that  of  starch,  (C6H1005)n,  with  the  n,  however,  represent- 
ing a  different  but  unknown  value. 

Although  cell-walls  when  young  consist  only  of  cellulose,  in 
some  structures  they  become  penetrated  later  by  other  materials 
which  are  probably  formed  by  alteration  of 
the  cellulose  itself,  and  which  give  new  proper- 
ties to  the  walls.  Thus,  it  is  a  stiffening  sub- 
stance called  lignin,  added  to  cellulose  walls, 
which  converts  them  into  wood,  and  also 
forms  other  hard  tissues,  such  as  the  shells  of 
nuts;  while  a  very  different  substance,  cutin 
or  suberin,  makes  the  walls  thoroughly  water- 
proof, as  they  are  in  all  cork,  and  in  the  thin 
waterproof  epidermis  which  ensheaths  the  en- 

FIG.    36.— A    cell    with 

tire  plant.     An  alteration  of  the  cellulose  of      parts  of  four  others, 

,,          ,  .      i  j  ,,  .,  from    the    interior   of 

another  kind  produces  the  mucilaginous  ma-  the  nut  of  ivory  Palm, 
terial  displayed  when  some  seeds  (e.  g.,  those  t°nSj  ££££ 
of  the  Flax),  are  placed  in  water,  or  when  deposition  of  layers 

of    cellulose,    through 

fallen  leaves  turn  gummy  on  sidewalks  in  wet,      which  run  canals  Per- 

.1  i  i        i        •      i-i  mitting  a  continuity  of 

warm,  autumn  weather;  and  such  also  is  the  protoplasm  from  one 
origin  of  the  mucilage  or  slime  found  in  des-  cel1  cavityto  auother- 
ert  plants  on  the  one  hand  and  water  plants  on  the  other,  with 
peculiar  functions  in  those  plants  to  be  later  considered. 

There  are  highly  consequential  facts  of  another  kind  about 
cellulose  whether  lignified  or  not.  It  burns  readily  in  presence 
of  oxygen,  being  converted  back  in  the  process  to  carbon  dioxide 
and  water,  the  very  substances  from  which  it  was  originally  made. 
When,  however,  it  is  subjected  for  a  long  period  of  time  to  pres- 
sure and  heat,  gradually  it  undergoes  definite  chemical  changes 
through  which  its  hydrogen  and  oxygen  are  removed,  leaving 
behind  the  solid  and  non- volatile  carbon.  This  is  exactly  what 
has  happened  in  the  case  of  the  plants  which  grew  of  old  time  in 


ii4  The  Living  Plant 

the  swamps  of  the  Coal  Period;  their  walls,  losing  the  oxygen  and 
hydrogen,  have  become  proportionally  richer  in  carbon  and  in- 
cidentally darker  in  color,  passing  gradually  through  stages  repre- 
sented by  peat,  lignite,  soft  coal  and  anthracite,  which  latter  is 
almost  entirely  carbon.  It  is  thus  that  our  beds  of  coal  have 
been  formed.  Somewhat  the  same  thing  occurs,  through  the  action 
of  heat,  in  the  charring  of  wood,  and  a  similar  process  produces 
the  black  humus  of  good  soils  from  roots  and  the  like.  When  the 
carbon  of  coal  remains  yet  longer  exposed  to  suitable  conditions, 
it  becomes  graphite  or  black  lead,  while  if  crystallized  it  forms 
diamond,  the  end  of  the  series.  And  it  is  interesting  to  note  in 
this  connection  that  we  do  riot  yet  know  any  natural  way  by  which 
pure  carbon  can  be  isolated  from  oxygen  without  photosynthesis 
constituting  a  step  in  the  process.  If  one  were  to  burn  the  dia- 
mond, he  would  form  carbon  dioxide  again,  and  thus  close  the 
chain  of  transformations  through  which  the  carbon  has  gone  since 
it  was  absorbed  from  the  air  by  a  living  green  plant  long  ages  ago. 
And  as  to  this  burning,  it  is  interesting  to  reflect  that  the  heat  and 
light  released  in  the  combustion  of  coal  is  energy  that  was  rendered 
latent  by  the  photosynthetic  dissociation  of  carbon  dioxide  when 
the  coal  was  first  formed  as  a  photosynthate;  it  has  been  kept 
stored  all  this  time  in  the  unsatisfied  affinity  of  its  carbon  for 
oxygen;  and  when  released  in  our  midwinter  fires,  it  is  really  the 
heat  arid  the  light  of  the  ancient  carboniferous  sun  that  is  warm- 
ing and  cheering  us. 

Gums.  These  are  solid  but  very  elastic  sweet  substances,  of 
which  gum  arabic,  used  in  gumdrops  and  on  postage  stamps, 
is  the  most  familiar  example;  the  gum  of  cherry  trees  is  another, 
and  the  substance  of  marsh  mallows  another,  though  the  spruce 
gum,  of  the  woods  and  the  schoolroom,  is  quite  different  as  will 
be  noted  below  under  resins.  These  gums  are  accumulated  in 
rifts  of  the  tissues  of  some  trees,  but  it  is  not  at  all  clear  why  the 
plant  should  make  them,  though  apparently  they  serve  at  times 
as  reserve  food.  Chemically  they  have  the  formula  (C6H10O5)n, 


The  Various  Substances  Made  by  Plants  115 

the  same  as  that  for  cellulose  and  starch,  but  with  n  meaning 
another  figure;  and  they  are  formed  no  doubt  from  grape  sugar, 
(probably  via  the  mucilaginous  modification  of  cellulose  men- 
tioned in  the  preceding  paragraph),  to  which  they  are  readily 
digested  back  by  both  plants  and  animals. 

Fruit-Jellies.  These  substances  are  familiar  to  all  housekeepers 
as  the  jelly  which  forms  when  fruits  or  vegetables  are  cooked 
(e.  g.,  grape  jelly,  orange  marmalade,  pumpkin  preserves),  though 
it  must  be  remembered  that  gelatine,  from  which  the  jellies  of  the 
tea-table  are  made,  is  an  animal  product.  In  the  living  plant 
they  are  solid,  being  insoluble  in  cold  water;  but  they  are  dissolved 
by  hot  water,  which  explains  why  they  appear  after  cooking. 
They  represent,  it  is  believed,  another  form  of  reserve  food.  Chem- 
ically they  are  known  as  pectins,  and  they  have  also  the  same 
general  formula  as  starch  (C6H1005)n.  They  are  formed  without 
doubt  from  grape  sugar  to  which  they  are  easily  digested  back. 

In  reading  this  account  of  these  various  carbohydrates,  two 
questions  will  inevitably  arise  in  the  mind  of  the  reader.  First, 
he  will  ask  how  it  is  possible  that  substances  with  properties  so 
different  as  those  of  starch,  cellulose,  gums  and  jellies  can  have 
the  same  chemical  composition.  The  answer  is  this,  that  on  the 
one  hand  the  letter  n  in  these  formula  represents  without  doubt 
a  different  number  in  each  case,  and  hence  the  composition  is  not 
really  identical,  while  on  the  other,  even  an  identical  formula 
can  be  associated  with  very  different  properties,  because  the 
properties  depend  not  only  on  the  elements  present,  but  upon 
the  way  these  elements  are  arranged  in  the  molecule;  and  they 
can  be  arranged  in  very  different  ways.  The  differences  between 
grape  sugar  and  fruit  sugar  are  wholly  of  this  latter  kind.  The 
second  question  the  reader  will  wish  answered  is  this, — why  do 
some  plants  store  up  their  reserve  food  in  the  form  of  sugar,  some 
as  starch,  others  as  cellulose,  and  others  as  oil,  soon  to  be  men- 
tioned. This  question  we  cannot  yet  answer  with  certainty,  but 
probably  the  general  explanation  offered  in  Chapter  III  for  the 


n6  The  Living  Plant 

diverse  ways  in  which  plants  develop  the  same  organs,  applies  to 
the  present  matter,  also, — namely,  the  plant  makes  the  form  of 
food  easiest  chemically  for  it  to  construct,  provided  of  course 
there  is  no  ecological  reason  for  making  one  kind  rather  than 
another. 

Class  m.    The  Secretions,  or  Derivatives  of  Carbohydrates 

So  heterogeneous  are  these  substances  in  composition,  proper- 
ties, and  uses,  that  they  are  held  in  one  class  by  hardly  any 
stronger  bond  than  that,  while  including  the  elements  of  Class  II, 
they  do  not  belong  therein.  Nor  is  the  name  which  I  give  them  a 
good  one,  for  they  include  some  things  which  are  not  truly  secre- 
tions, while  not  all  of  the  secretions  are  included  in  this  class;  but 
I  can  think  of  no  better  general  designation.  The  principal 
members  are  the  following. 

Plant  Oils.  These  are  of  two  distinct  kinds.  First,  are  the  fixed 
oils,  which  are  properly  plant  fats,  familiar  to  us  hi  the  various 
oils  used  in  food  or  in  medicine,  notably  olive  oil,  castor  oil,  cot- 
ton seed  oil.  They  occur  rather  widely  scattered  in  plants,  as 
tiny  isolated  drops,  scattered  through  the  protoplasm;  but  they 
accumulate  in  quantity  in  many  kinds  of  seeds,  including  nuts, 
to  which  they  give  a  distinctive  oily  luster,  and  in  which  they  act 
very  obviously  as  a  reserve  food  for  the  use  of  the  embryo  in 
germination.  A  reason  why  oil  is  stored  in  seeds  more  frequently 
than  elsewhere  has  been  found  in  a  linking  of  two  facts; — first, 
food  value  for  food  value,  oil  is  a  much  lighter  substance  than  any 
other  kind  of  food  stored  by  plants;  and  second,  the  seeds  storing 
it  are  mostly  disseminated  by  the  wind  and  hence  need  to  be  kept 
just  as  light  in  weight  as  possible.  And  with  these  oils  as  with 
other  substances,  good  food  for  plants  is  good  food  for  animals 
also,  the  food  needs  of  both  being  closely  alike.  Chemically 
these  fats  are  rather  complex,  a  typical  formula  being  C57H11006, 
which  shows  that  they  are  markedly  poor  in  oxygen;  and  herein 
lies  the  reason  why  plenty  of  fresh  air  is  needed  for  their  assimila- 


The  Various  Substances  Made  by  Plants  117 

tion  by  man.  They  are  formed  in  living  cells  from  starch,  and 
therefore  ultimately  from  grape  sugar,  to  which  they  can  be 
changed  back  in  germination  and  digestion  by  the  action  of  suit- 
able enzymes. 

Related  to  the  fats  in  some  respects,  though  to  the  later- 
described  proteins  in  others,  are  the  lecithins,  widely  distributed 
in  plants,  and  possessing  a  considerable  interest  as  the  probable 
basis  for  the  formation  of  the  vastly-important  and  complicated 
substance  chlorophyll,  the  composition  of  which,  aside  from  the 
presence  of  carbon,  hydrogen,  oxygen  and  phosphorus,  is  still 
rather  uncertain. 

The  other  kind  of  oils, — the  ethereal,  essential,  or  volatile 
oils,  are  very  different  in  composition  and  meaning.  They  are 
familiar  to  us  chiefly  in  the  fragrant  oil  of  lemon  and  oil  of  cloves, 
and  are  the  causes  also  of  the  odors,  sometimes  fragrant  and 
sometimes  acrid,  of  many  kinds  of  leaves  (e.  g.,  Lemon  Geranium) 
when  cut  or  crushed;  and  they  cause  likewise  the  fragrance  of 
flowers  and  fruits.  Camphor,  and  some  other  aromatic  materials 
are  related  substances.  They  are  not  food  products,  as  the  fats 
are,  but  serve  mostly  ecological  uses,  either  in  connection  with 
the  protection  of  plants  against  insects  or  Fungi,  or  for  the  at- 
traction of  animals  in  connection  with  dissemination  of  seeds 
and  cross  pollination  of  flowers,  as  we  shall  later  consider  in  detail 
along  with  those  respective  subjects.  They  are  stored  as  a  rule 
in  special  receptacles  or  glands,  often  of  considerable  size  (figure 
37).  Chemically  they  are  most  diverse,  some  of  them  consisting 
only  of  carbon  and  hydrogen,  approaching  near  to  the  formula 
C10H16.  Little  is  known  as  to  their  exact  mode  of  formation. 

It  is  a  non- volatile  oil  (called  toxicodendrol) ,  which  is  the 
poisonous  susbtance  in  the  Poison  Ivy;  and  the  fact  that  it  is  a 
non-evaporating  oil  explains  why  it  is  so  very  difficult  to  remove 
from  the  skin,  and  why  it  persists  in  plants  which  are  long  dead 
and  dried. 

Plant  Acids.  These  are  agreeably  familiar  to  us  as  the  sub- 


u8 


The  Living  Plant 


stances  which  give  the  pleasant  acid  taste  to  fruits.  Thus,  malic 
acid  gives  the  tart  taste  to  apples  and  currants,  citric  acid  to 
lemons  and  oranges,  tartaric  acid  (from  which  cream  of  tartar 

is  made)  to  grapes.  In  all  of  these 
cases  there  is  a  reason,  as  our 
chapter  on  Dissemination  will 
show,  why  these  fruits  should  be 
eaten  by  animals,  to  which  the 
acids  certainly  serve  to  render  the 
fruits  more  attractive.  On  the 
other  hand  tannic  acid,  which  oc- 
curs in  the  bark  of  many  plants, 
(and  from  which  man  extracts  it 
FIG.  37.-A  gland,  highly  magnified,  for  tanning  leather),  has  an  as- 
formed  by  a )  fusion  of  several  ceils  tringent  taste  unpleasant  to  ani- 

contammg  a  large  drop  of  an  ethereal  *• 

oil,  as  seen  in  a  cross-section  of  a  leaf    mals,    agaUlSt    which,    accordingly, 

of  Dictamnus  Fraxinella. 

its  presence  has  some  tendency  to 

protect  the  plant  tissues.  These  acids,  which  all  occur  in  solu- 
tion in  the  sap,  have  a  comparatively  simple  composition,  the 
formula  of  malic  acid,  for  example,  being  C4H605.  Their  mode 
of  formation  is  not  entirely  understood. 

Plant  Waxes.  These  occur  cliiefly  on  the  surface  of  plants, 
where  they  constitute  the  bloom,  commonly  of  bluish  color,  which 
is  familiar  upon  plums  and  some  leaves.  On  the  dry  berries  of 
the  Bayberry,  a  common  plant  of  the  coast,  a  wax  accumulates 
in  such  quantity  that  in  early  days  it  was  gathered  and  used  for 
the  making  of  candles.  In  general  the  waxes  seem  to  render 
plants  immune  against  wetting,  after  the  manner  of  the  oil  on 
the  back  of  the  proverbial  duck, — the  disadvantage  of  the  wet- 
ting being  this,  that  the  water  would  clog  the  stomata,  and  hence 
prevent  the  passage  of  gases  that  are  needed  in  photosynthesis. 
If,  now,  the  reader  should  ask  me  why,  when  the  wax  is  thus  of 
advantage,  so  many  plants  do  not  have  it,  I  would  answer  by 
asking  in  turn  why  it  is,  that,  if  riches  are  such  an  advantage  (or 


The  Various  Substances  Made  by  Plants  119 

at  least  are  commonly  thus  reckoned),  so  few  men  possess  them. 
The  reason  I  take  to  be  fundamentally  the  same  in  both  cases; — 
some  kinds  never  get  the  right  start  towards  constructing  them, 
or  else  have  not  the  capacity  to  manufacture  them.  Chemically 
the  waxes  are  very  closely  related  to  the  oils,  and  no  doubt  are 
built  up  in  the  same  general  way. 

Resins.  Under  this  name  falls  a  variety  of  substances  of  which 
typical  examples  are  familiar  in  the  balsam  of  the  Fir  and  the 
Pine;  in  spruce  gum;  and  in  rosin;  myrrh  and  frankincense  are 
others;  and  much  of  the  milky  juice  (or  latex)  of  plants,  from  which 
the  rubber  of  commerce  is  made,  is  composed  of  resins  or  closely 
related  substances.  Chemically  the  resins  are  most  diverse,  and 
their  mode  of  origin  is  as  little  understood  as  is  their  function  in 
the  plant.  They  are  usually  accumulated  in  special  passages, 
from  which  they  sometimes  flow  out  at  a  break  (e.  g.,  in  Pines), 
in  a  way  to  suggest  that  they  serve  as  a  temporary  salve, — a  kind 
of  first  aid  to  an  injury.  At  times  they  appear  to  be  utilized  as 
food,  which  is  likely  enough,  since  there  is  every  reason  to  sup- 
pose that  plants,  precisely  like  animals,  when  driven  by  hunger, 
will  resort  to  the  use  of  materials  which  they  would  otherwise  re- 
ject with  disdain. 

Glucosides.  These  substances  are  more  interesting  than  con- 
spicuous, the  most  familiar  being  that  called  amygdalin,  which 
gives  the  bitter  taste  to  seeds  of  almonds  and  apples;  while  the 
peppery  taste,  so  common  in  plants  of  the  mustard  family,  is 
also  due  to  a  glucoside.  Their  meaning  in  the  plants  is  not  known, 
although  they  may  find  some  incidental  service  in  protecting 
against  animals  the  parts  which  possess  them.  With  the  gluco- 
sides  belong  also  some  of  the  brightest  coloring  matters  produced 
by  plants,  including  the  red  dye  madder  and  the  blue  dye  indigo. 
Here  also  comes  erythrophyll  (called  also  anthocyan),  that  red 
color  with  which  we  have  made  pleasant  acquaintance  already  as 
giving  brilliant  hues  to  ripened  fruits,  and  the  glory  to  the  fo- 
liage of  autumn.  Chemically  the  glucosides  owe  their  name  to 


120  The  Living  Plant 

the  fact  that  they  are  compounds  of  glucose  with  some  one  or 
more  definite  substances,  into  which  they  can  again  be  broken 
up.  Some  of  them  contain  nitrogen,  as  for  instance  the  amyg- 
dalin  above  mentioned  (its  formula  is  C20H27OnN),  which  allies 
them  in  some  measure  with  the  nitrogen-containing  substances 
next  to  be  considered,  especially  the  alkaloids. 

Class  IV.    The  Nitrogen-Assimilates,  or  Amides 

These  substances,  dissolved  in  the  sap  of  plants  and  having 
no  particular  uses  to  us,  are  not  commonly  known;  but  they  are 
vastly  important  nevertheless,  inasmuch  as  they  constitute  the 
connecting  step  between  the  carbohydrates  and  the  indispensable 
proteins,  soon  to  be  considered.  The  commonest  is  asparagin, 
dissolved  in  the  sap  of  young  asparagus  plants,  from  which  it  can 
easily  be  crystallized  out.  Its  formula,  typical  of  the  group, 
is  C4H8O3N2,  which  shows  the  presence  of  the  nitrogen  along  with 
the  elements  of  carbohydrates;  and  there  is  no  doubt  that  the 
ultimate  source  of  the  materials  is  the  photosynthetic  grape  sugar 
together  with  nitrogen  from  compounds  absorbed  with  water  by 
the  roots.  The  amides  are  not  known  to  perform  any  special  func- 
tion of  their  own  in  the  plant,  and  probably  find  their  significance 
simply  as  a  necessary  chemical  step  in  the  formation  of  proteins. 

The  incorporation  of  nitrogen  with  the  elements  of  the  car- 
bohydrates is  a  step  of  the  first  biological  magnitude,  since  the 
nitrogen  is  the  most  essential  and  distinctive  additional  con- 
stituent of  the  most  important  of  all  biological  substances, — 
living  protoplasm.  We  have  already  considered,  (in  Chapter 
II),  the  source  of  the  plant's  supply  of  carbon,  oxygen,  and  hydro- 
gen, and  must  now  turn  aside  from  our  main  theme  to  examine 
the  source  of  the  nitrogen  supply,  a  subject  all  the  more  important 
because  of  the  fundamental  economic  bearings  it  has.  Nitrogen, 
it  should  be  needless  to  recall  to  the  reader,  is  the  colorless  gas 
which  makes  up  very  nearly  four-fifths  of  the  atmosphere;  and 
from  such  an  abundance  plants  ought  apparently  to  have  no 


The  Various  Substances  Made  by  Plants  121 

difficulty  in  drawing  all  that  they  need.  As  a  matter  of  fact, 
however,  the  typical  plants  take  no  nitrogen  at  all  from  the  air, 
even  starving  to  death  for  want  of  a  little  while  bathed  in  this 
lavish  abundance;  and  the  reason  they  do  not  is  that  they  cannot. 
The  most  prominent  characteristic  of  nitrogen  is  its  chemical 
inertness,  or  reluctance  to  enter  into  combination  with  any  other 
substances, — a  circumstance,  indeed,  to  which  its  abundance 
in  the  atmosphere  is  due;  and  its  union  with  oxygen  or  other 
substances  can  be  effected  only  by  the  agency  of  electric  sparking 
machines,  or  other  methods  involving  the  expenditure  of  high  ten- 
sion energy.  Now  our  typical  large  plants  have  not  in  their  struc- 
ture any  equivalent  for  sparking  machines  or  other  arrangements 
releasing  suitable  energy,  although,  as  will  presently  appear,  the 
lowly  Bacteria  seem  better  provided  in  this  particular.  Since  they 
cannot  make  use  of  the  free  nitrogen  of  the  air,  plants  have  had  to 
resort  to  the  only  other  possible  source  of  supply,  viz.,  substances 
in  the  soil  containing  it  already  combined,  which  substances, 
moreover,  must  be  soluble  in  water  to  admit  of  their  absorption 
by  the  roots.  The  compounds  called  nitrates  best  meet  these 
conditions,  and  they,  accordingly,  are  the  source  of  most  of  the 
nitrogen  which,  with  appropriate  intermediate  chemical  steps, 
is  combined  with  the  elements  of  the  carbohydrates  to  form 
amides. 

If  nitrates  were  as  plenty  in  soils  as  plants  could  make  use  of, 
then  our  digression  in  pursuit  of  this  substance  could  end  right 
here.  But  in  fact  the  nitrates  in  most  soils  are  so  scant  that  the 
majority  of  plants  live  all  the  time  in  touch  with  nitrogen  scarcity, 
and  this  is  one  of  the  chief  of  the  factors  which  limit  the  luxuriance 
of  their  growth  and  expansion.  It  is,  perhaps,  worth  noting  in 
passing,  that  especial  scarcity  of  nitrogen  in  some  situations 
is  correlated  with  an  insectivorous  habit  in  plants  which  reside 
there, — the  advantage  of  this  habit  consisting  in  the  abundance 
of  combined  nitrogen  obtainable  by  digestion  from  the  bodies  of 
insects.  A  chief  reason  for  the  scarcity  of  nitrates  in  the  soil  lies 


122  The  Living  Plant 

in  that  very  solubility  which  renders  them  absorbable  by  plants, 
for  it  leads  to  their  constant  drainage  away  with  the  superfluous 
water;  and  were  it  not  for  a  constant  renewal  of  the  nitrate  sup- 
ply plant  life  would  soon  be  starved  to  extinction.  This  renewal, 
known  as  the  nitrification  of  soils,  is  a  matter  of  such  biological 
and  economic  consequence  that  we  must  now  consider  it  with 
some  care. 

The  natural  nitrification  of  soils  takes  place  in  four  ways. 
First,  there  is  a  constant  return  of  combined  nitrogen  to  the  soil 
from  the  excretions  of  animals,  and  the  decay  of  plant  and  animal 
bodies.  Second,  a  small  amount  of  combined  nitrogen  is  added 
to  the  soil  with  the  rain  which  falls  during  thunder  showers,  for  the 
lightning  acts  as  a  kind  of  gigantic  natural  sparking  machine  which 
forces  the  nitrogen  and  oxygen  of  the  air  into  combination; 
thus  is  formed  the  soluble  nitrous  acid,  which  is  caught  and  taken 
into  the  soil  by  the  rain.  Third,  nitrates  are  constantly  though 
slowly  added  to  the  soil  by  the  natural  decay  of  the  rocks  which 
contain  them.  In  moist  climates  they  must  drain  away  about  as 
fast  as  they  are  formed,  but  in  dry  climates  the  drainage  is  slower 
than  their  formation  and  they  accumulate  in  the  soil.  This  is 
a  reason  for  the  richness  of  the  finer  soils  of  the  deserts,  which 
blossom  as  the  rose  when  water  is  added  by  aid  of  irrigation. 
Fourth  (and  far  the  most  important)  of  the  natural  methods  of 
soil  nitrification  is  bacterial  activity.  Everybody  knows  that  a 
soil  in  order  to  be  rich  must  contain  a  proportion  of  humus, 
the  material  which  is  dark  in  color  and  supplies  the  open  char- 
acter. This  humus  consists  chiefly  of  decaying  vegetable  matter, 
which  provides  both  the  home  and  the  nourishment  for  countless 
numbers  of  tiny  organisms,  chiefly  Molds  and  Bacteria.  These 
Bacteria,  popularly  known  as  Germs,  are  of  several  kinds,  of 
which  some,  in  the  course  of  their  own  processes,  incidentally 
work  over  the  less  valuable  nitrogen  compounds  of  the  soil  to 
more  valuable  ones,  while  still  others,  and  these  the  most  impor- 
tant, actually  force  the  nitrogen  and  oxygen  of  the  air  to  unite 


The  Various  Substances  Made  by  Plants  123 

into  the  simple  compounds  which  later  are  worked  up  to  nitrates 
by  the  others.  It  is  not  yet  known  how  these  Bacteria  accomplish 
this  crucial  first  step  of  nitrification,  but  the  source  of  the  energy 
is  plain;  it  is  supplied  by  their  intense  respiratory  power,  in  which 
they  surpass  some  hundred-fold  the  larger  plants.  This  fact  of 
the  nitrification  of  soils  through  the  activity  of  Bacteria  is  one 
of  the  most  important  in  nature. 

It  may  here  occur  to  the  practically-minded  reader  to  ask 
whether  this  power  of  Bacteria  to  add  nitrogen  compounds  to 
soils  cannot  be  utilized  artificially  for  the  enrichment  of  poor  soils. 
It  can  be,  and  to  some  extent,  has  been;  and  living  Bacteria 
of  the  suitable  sorts  have  actually  been  multiplied  and  distributed 
for  trial  by  our  own  Department  of  Agriculture,  and  have  been 
offered  for  sale  to  farmers  both  in  Europe  and  America,  though 
the  process  is  not  as  yet  a  commercial  success.  However,  in  the 
utilization  of  the  nitrifying  Bacteria  man  was  long  anticipated 
by  at  least  one  great  group  of  Plants,  the  Pea  Family,  or  Legu- 
minosaB,  the  members  of  which  have  actually  colonized  the  nitrify- 
ing Bacteria  upon  their  own  roots,  thus  making  sure  that  the  en- 
tire product  of  the  Bacteria  shall  be  available  to  themselves 
without  any  loss  through  drainage  or  use  by  other  plants.  Most 
people  have  seen  upon  the  roots  of  Peas,  Beans,  and  others  of 
this  family,  the  wart-like  or  pea-like  swellings,  whose  appearance 
is  well  shown  in  the  accompanying  photograph,  (figure  38). 
These  nodules  are  residences  inside  the  plants  occupied  by  the 
Bacteria.  The  connection  is  mutually  beneficial,  for  the  Bacteria 
receive  carbohydrates  from  the  green  plants  which  receive  nitrog- 
enous compounds  from  them.  It  is  because  of  the  efficiency  of 
this  arrangement  that  the  seeds  of  plants  in  the  Pea  Family  are 
richer  in  nitrogenous  food  substances  than  any  others;  and  this 
latter  fact  in  its  turn  explains  why  Peas  and  Beans  are  the  best 
of  all  plant  substitutes  for  meat,  which  is  mostly  protein.  This 
relative  richness  of  Leguminosa?  in  nitrogenous  compounds  ex- 
plains also  the  reason  underlying  the  ancient  farming  practice 


124 


The  Living  Plant 


of  green-manuring,  that  is,  plowing  in  Clover  and  other  legumi- 
nous crops  to  enrich  the  soil.    It  is  from  these  same  nodules,  also, 
that  the  Bacteria  have  been  taken  and  grown  for  the  commercial 
enrichment  of  the  soil,  as  mentioned  on  the  preceding  page. 
In  our  consideration  of  these  four  natural  methods  of  soil 


FIG.  38. — The  roots  of  several  Bean  plants,  photographed  about  half  the  natural  size, 
showing  the  collections  of  wart-like  nodules  which  contain  the  nitrifying  Bacteria. 

nitrification  we  must  not  forget  the  artificial  aid  of  man,  who, 
for  his  own  purposes,  adds  to  the  soil  both  chemical  fertilizers 
and  barnyard  manures,  with  their  rich  supplies  of  nitrogenous 
and  other  compounds. 

Finally  it  is  important  to  note  that  the  plants,  for  their  part, 
have  a  way  of  meeting  the  nitrogen  scarcity  of  soils, — viz.,  they 


The  Various  Substances  Made  by  Plants  125 

waste  none.  To  this  end  they  even  go  so  far  as  to  remove  from 
their  leaves,  before  these  are  dropped,  such  of  the  nitrogenous 
and  other  compounds  as  can  be  used  economically  again.  Unlike 
animals,  they  excrete  no  nitrogen,  or  extremely  little,  in  either 
solid,  liquid,  or  gaseous  form,  but  conserve  it  with  care  and  use 
it  over  and  over  again;  so  that  it  is  only  released  in  the  end  by 
their  decay  after  death. 

Class  V.     The  Principal  Poisons,  or  Alkaloids 

These  substances  are  notorious  as  including  the  most  violent 
plant  poisons.  Thus  strychnine  (from  the  Strychnos  bean), 
nicotine  (from  the  Tobacco  leaves),  morphine  (from  the  milky 
juice  of  Poppies)  are  alkaloids,  as  is  the  poison,  muscarine,  of  the 
deadliest  Mushrooms.  Some  alkaloids,  while  not  poisonous, 
have  strong  properties  in  other  respects,  such  as  quinine,  obtained 
from  the  bark  of  the  Cinchona  tree  and  efficacious  in  breaking  up 
fevers;  caffeine,  the  stimulating  substance  in  Tea  leaves  and  Coffee 
berries;  cocaine  from  Cocoa  seeds,  the  well  known  local  anesthetic 
and  fatally-alluring  drug.  Their  meaning  in  the  plant  is  uncer- 
tain, and  all  the  more  puzzling  since  they  mostly  are  poisonous 
to  the  very  plants  which  produce  them  if  injected  into  other 
parts  of  their  tissues.  Nor  is  it  certain  just  how  they  produce 
their  poisonous  effects.  Alkaloids  occur  also  in  animal  tissues 
as  a  product  of  the  processes  of  fermentation  and  decay;  they  are 
called  ptomaines,  and  are  very  deadly,  being  the  real  cause  of 
death  in  bacterial  diseases.  Chemically  the  alkaloids  are  related 
to  the  amides,  from  which  they  are  no  doubt  formed,  not  at  all 
as  a  step  in  the  formation  of  proteins,  but  as  a  side  group.  A 
typical  formula  is  that  of  caffeine,  C8H1002N4. 

It  has  recently  been  discovered  that  the  roots  of  our  common 
field  crops  appear  to  excrete  into  the  soil  minute  quantities  of 
substances  poisonous  to  the  plants  which  produce  them;  and  it  is 
probable  that  the  presence  of  such  substances,  and  not  the  ex- 
haustion of  the  necessary  mineral  matters,  is  the  real  cause  of 


126  The  Living  Plant 

the  sterility  of  some  soils,  which  are  therefore  "poisoned"  rather 
than  "exhausted."  The  composition  of  these  substances  is  not 
known,  except  that  they  are  complicated  and  perhaps  nitrog- 
enous, in  which  case  they  may  be  found  to  belong  with  this 
group  of  the  alkaloids. 

The  reader  will  recall  that  the  active  properties  of  the  alka- 
loids were  somewhat  foreshadowed  in  the  nitrogenous  glucosides, 
and  later  he  will  also  make  acquaintance  with  remarkably  active 
properties  of  another  kind  which  characterize  not  only  the  pro- 
teins entering  into  living  protoplasm,  but  also  the  enzymes,  with 
their  very  striking  chemical  powers.  The  common  feature  which 
distinguishes  all  of  these  substances  in  contrast  with  the  more 
passive  groups  is  the  possession  of  nitrogen,  which  seems  there- 
fore to  be  associated  with  the  most  active  properties  in  plant 
substances.  This  fact  is  sufficiently  curious  in  face  of  the  chemical 
inertness  of  nitrogen,  and  one  can  fancy  this  element  as  reluctant 
to  enter  into  combinations,  restless,  so  to  speak,  while  in  them, 
and  making  disturbance  in  its  efforts  to  escape  to  its  original 
freedom. 

Class  VI.    The  Flesh-Formers,  or  Proteins 

These  are  the  most  important  substances  made  by  plants, 
entering  as  they  do  into  the  composition  of  living  protoplasm. 
They  are  more  familiar  in  animals  than  in  plants,  for  flesh  is 
made  up  of  them;  but  they  are  distributed  throughout  the  living 
parts  of  all  plants,  either  in  the  active  protoplasm  or  stored  as 
reserve  food,  especially  in  seeds.  They  are  vast  in  number,  elab- 
orate in  composition,  and  only  imperfectly  known.  Chemically 
they  are  distinguished  from  all  of  the  preceding  groups  by  con- 
taining not  only  the  elements  of  the  latter,  but  also  sulphur, 
while  some  of  them  possess  phosphorus  too,  so  that  their  com- 
position may  thus  be  expressed  CnHnOnNnSn(Pn).  Some  of  their 
molecules  are  of  very  great  complexity ;  thus,  there  is  an  albumin 
with  the  formula  C72oH1134N218O248S5;  and  there  are  other  proteins 


The  Various  Substances  Made  by  Plants  127 


in  which  the  elements,  or  atoms,  of  the  molecule,  must  sum  up  to 

more  than  15,000,  or  even,  in  some  cases,  more  than  30,000.   Many 

of  these  substances  differ  little  from  one  another  in  properties, 

and  moreover  are  readily  convertible  one  into 

another;  and  the  facts  seem  to  indicate  that 

these  elaborate  forms  are  really  multiples  (or 

polymers)   of  some   simple   protein  molecule, 

built  up  in  the  same  manner  as  are  starch  and 

cellulose  from  a  simple  carbohydrate  molecule. 

Nor  is  it  to  be  supposed  that  all  of  these  sub- 

stances have  each  a  separate  meaning  in  the 

plant,  though  they  may  have;  but  many  of 

them  no  doubt  are  simply  manifestations  of 

chemical   individuality  in   the   plant,   as  the 

forms  of  starch  grains  are  manifestations  of 

physical  individuality. 

Several  different  groups  of  Proteins  are  recognized  by  chemists, 

of  which  I  shall  here  mention,  even  though  in  little  more  than  the 
Homeric  fashion,  the  more  important.  They 
are,  —  Albumins,  substances  like  white  of  egg, 
thinly  spread  through  many  plants:  Globulins, 
which  form  definite  grains  in  some  seeds  like 
Corn  (figure  39),  and  beautiful  crystals  in  Castor 
Bean,  Potato  and  some  other  plants  (figure  40)  : 
Glutelins,  typified  by  the  familiar  gluten  of 

fl°UI>    wMch    &VeS    the  aggluthlOSity   to    dough  : 


FIG.  39.— A  cell,  highly 
magnified,  from  the 
proteinaceous  layer 
just  under  the  husk 
of  Corn,  showing  nu- 
merous protein 
grains  interspersed 
with  a  few  starch 
(larger)  grains,  all 
embedded  in  living 
protoplasm. 


interior  of  a  Castor  Prolamins  especially  distinctive  of  the  seeds  of 

Bean,    showing    the  .  .     .  , 

crystalline    protein  grams  :  N  ucleo-proteins  ,  containing  phosphorus, 


of  cells:  Phosphoproteins  (called  also 


tur^rendered  some-  and  forming  the  chromosome  substance  of  the 

what  clearer  by 
treatment  with  re- 
agents), embedded  in  albuminates)  cheese-like  materials  found  in 

living  protoplasm. 

some  seeds:  Proteases  and  Peptones,  very  im- 
portant because  they  are  the  soluble  and  diffusable  proteins  into 
which  the  insoluble  kinds  are  converted  in  digestion  by  the 


128  The  Living  Plant 

action  of  peptonizing  enzymes :  and  there  are  others  likewise  of 
rarer  sort  and  lesser  consequence. 

The  mention  of  the  presence  of  sulphur  and  phosphorus  in 
proteins  will  lead  the  reader  to  inquire  for  the  source  of  supply 
of  those  elements.  The  answer  is  ready.  They  are  derived  from 
soluble  sulphates  and  phosphates  absorbed  from  the  soil  by  the 
roots,  and  are  incorporated,  through  chemical  reactions  still 
imperfectly  known,  with  the  elements  contained  in  the  amides. 
All  soils  contain  all  of  the  sulphates  that  plants  need,  and  usually 
all  of  the  phosphates,  though  at  times  the  latter  are  insufficient, 
and  must  be  added  as  fertilizers  to  ensure  good  crops. 

Class  VII.     The  Regulators  of  Metabolism,  or  Enzymes 

It  is  safe  to  say  that  the  enzymes  (called  also  ferments)  are 
the  most  remarkable  and  least  known,  although  among  the  most 
important,  of  all  substances  produced  by  plants, — or  by  animals, 
either.  They  are  characterized  by  this  remarkable  power, — viz., 
they  can  cause  chemical  changes,  each  of  one  definite  kind,  in 
other  substances,  without  themselves  entering  into  the  reaction 
or  suffering  any  appreciable  alteration.  Because  of  this  mode 
of  action  very  small  quantities  of  enzymes  can  alter  chemically 
great  quantities  of  material.  Thus  the  enzyme  diastase,  which 
occurs  both  in  the  saliva  of  man  and  also  in  the  starch-storing 
organs  of  plants,  can  convert  (chemically,  hydrolyze)  great 
quantities  of  the  insoluble  starch  by  two  or  three  steps  into  grape 
sugar,  a  soluble  diffusable  material;  likewise  the  enzyme  protease 
(pepsin)  occurring  in  both  plants  and  animals,  hydrolyzes  in- 
soluble indiffusable  proteins  into  soluble  diffusable  peptones; 
also  the  enzyme  lipase  converts  insoluble  fats  into  soluble  fatty 
acids  and  glycerine:  cytase  converts  cellulose  of  Ivory  palm  and 
Date  into  soluble  sugars;  and  there  are  many  others  of  lesser 
prominence.  It  is  these  changes  which  constitute  digestion, 
whether  in  plants  or  in  animals.  By  aid  of  the  enzymes  the  plant 


The  Various  Substances  Made  by  Plants  129 

can  not  only  produce  and  control  chemical  changes  within  its 
own  body,  but,  by  pouring  them  out  in  suitable  places,  can  dissolve 
extraneous  materials  and  later  absorb  these  again  for  its  own  use. 
It  is  thus  that  insectivorous  plants  can  digest  the  insects  they 
capture;  parasites  can  penetrate  into  the  tissues  of  a  host;  and 
pollen  tubes  can  digest  their  way  down  the  solid  tissues  of  the 
style,  absorbing  the  digested  materials  for  use  in  their  own 
growth.  But  there  are  many  other  phases  of  enzyme  action  also; 
thus  the  unfermentable  cane  sugar  is  hydrolyzed  (or  inverted) 
to  fermentable  grape  sugar  by  invertase,  and  grape  sugar  is  fer- 
mented to  alcohol  and  carbon  dioxide  by  zymase,  produced  by  the 
Yeast  plant.  And  there  are  other  cases  innumerable  which  we 
cannot  take  space  to  consider. 

Chemically  and  physically  we  know  very  little  about  the  en- 
zymes, because  it  has  not  yet  been  found  possible  to  extract  them 
from  the  protoplasm  in  a  pure  state;  and  even  their  very  existence 
would  not  be  recognized  at  all  were  it  not  for  their  effects.  It 
is  not  even  certain  that  they  are  related  to  the  Proteins,  although 
there  is  indirect  evidence  pointing  that  way;  nor  are  we  sure  that 
they  are  liquids  thinly  saturating  the  protoplasm,  though  this 
seems  probable.  Still  less  is  it  known  how  they  produce  their 
remarkable  effects,  although  a  homologous  power  exists  in  those 
inorganic  substances  called  catalyzers.  Each  kind  can  produce 
only  one  chemical  change,  and  that  as  a  rule  but  a  slight  one, 
but  the  cooperation  of  several  can  cause  a  series  of  changes  large 
in  the  end;  and  it  may  be  true  that  they  cause  most,  if  not  indeed 
all,  of  the  chemical  processes  which  the  living  protoplasm  carries 
on.  They  are  the  tools,  so  to  speak,  with  which  the  protoplasm 
effects  the  chemical  results  it  requires.  Indeed  to  some  investi- 
gators it  has  seemed  likely  that  the  enzymes  are  the  principal 
material  bases  of  heredity,  and  that  the  chromosomes  of  the 
nuclei,  known  to  be  conveyors  of  heredity,  consist  chiefly  of  col- 
lections of  enzymes.  Truly  the  importance  of  the  enzymes  is 
great,  and  their  further  study  in  the  near  future  is  likely  to  throw 


130  The  Living  Plant 

much  light  upon  some  of  the  most  fundamental  problems  of  Bi- 
ology. 

Class  VIII.     Living  Protoplasm 

This  substance  is  of  such  importance  and  complexity  as  to  re- 
quire for  its  treatment,  a  separate  chapter,  which  follows.  It 
need  only  be  said  in  this  connection  that  so  far  as  chemical 
analysis  has  been  able  to  penetrate  into  the  mysteries  of  living 
protoplasm,  it  appears  to  be  merely  a  very  complicated  mixture 
of  proteins  with  many  simpler  substances.  Here  for  example 
is  a  list  of  the  substances  which  have  been  recognized  in  a  chemical 
analysis  of  the  protoplasm  of  one  of  the  lower  plants; — 

Water,  Pepsin  and  Myosin,  Vitellin,  Plastin,  Guanin,  Xanthin, 
Sarkin,  Ammonic  carbonate,  Asparagin  and  other  amides,  Pepton 
and  Peptonoid,  Lecithin,  Glycogen,  Aethalium  sugar,  Calcic  com- 
pounds of  higher  fatty  acids,  Calcic  formate,  Calcic  acetate,  Calcic 
carbonate,  Sodic  chloride,  Hydropotassic  phosphate,  Iron  phosphate, 
Ammonio-magnesic  phosphate,  Tricakic  phosphate,  Calcic  oxalate, 
Cholesterin,  Fatty  acids  extracted  by  ether,  Resinous  matter,  Glycerin, 
coloring  matter,  etc.,  Undetermined  matters. 

In  this  list,  which  I  give  in  order  to  illustrate  the  chemical 
complexity  of  protoplasm,  all  of  the  constituents  are  well-known 
substances,  no  one  of  which  has  any  of  the  properties  of  life, 
unless  such  a  substance  lies  hidden  in  the  trifling  amount  of  "  Un- 
determined matters" ;  nor  has  any  chemist  yet  been  able  to  identify 
any  distinctive  living  substance, — any  of  that  protoplasm  par 
excellence  which  we  are  logically  bound  to  believe  must  exist. 
But  the  further  consideration  of  this  subject  belongs  with  the 
next  chapter. 

Such  are  the  groups  of  substances  which  plants  build  upon  the 
foundation  laid  by  the  photosynthate.  We  may  summarize  their 
relationship  in  a  diagrammatic  manner,  after  the  analogy  of  a  tree 
of  ascent,  as  shown  herewith. 


The  Various  Substances  Made  by  Plants 


Living  Protoplasm 

^Enzymes 


Proteins 


Amides 


-Alkaloids 


It  may  perhaps  have  occurred  to  the 
reader  ere  this  to  inquire  what  proportion 
of  the  original  basal  photosynthate  is  used  in 
the  construction  of  each  of  these  classes  of 
substances.  The  question  is  a  fair  one  but 
difficult  to  answer,  partly  because  the  pro- 
portions would  be  so  different  with  the  vari- 
ous kinds  of  plants,  and  partly  because  we 
have  so  few  data  for  making  calculations. 
However,  it  is  possible  to  make  a  generaliza- 
tion for  plants  as  a  whole,  and  this  has  been 
done  in  the  table  below,  which,  although  lit- 
tle more  than  a  guess,  has  yet  some  value.  For  simplicity  I  have 
reduced  the  table  to  the  kinds  of  known  and  visible  substances, 
grouping  together  the  others  as  " special  substances";  and  inci- 
dentally I  have  added  the  ultimate  fate  of  the  various  groups. 


-Oils-Resins 


Carbohydrates 

I 
(Photosynthate) 


I 

o 


£ 


<£  ^  ^  ^  I  ^ 

»o  *o  o  *o  o 

«  «  -  -  8 

o  o  q  q  i  q 

tS  tq  a  &q 


8 

3 
^> 


o    o 
u    o 


o        5 

1  1 

a       'S, 


S  3 

CQ  >> 

—  ^ 

I  I 


d 
a 

U  .. 


10    :  ^ 
^ 


g^.9 

c3    w   -g 

8-3.1 


132 


The  Various  Substances  Made  by  Plants  133 

This  table  brings  out  clearly  once  more  that  most  fundamental 
of  facts  about  the  physical  constitution  of  living  things,  that  their 
substance  is  all  derived  originally  from  carbon  dioxide  and  water, 
with  a  few  minor  additions,  and  is 
all  returned  in  the  end  back  to  the 
same  source,  undergoing  en  route 
transformations  of  substance  and 
energy  which  constitute  the  princi- 
pal visible  phenomena  of  life.  The 
organism  is  made  up  of  a  little 
of  those  substances  temporarily 
withdrawn  from  the  general  cir- 
culation of  nature  and  interacting  FIG.  4i.— A  ceil,  highly  magnified,  from 

Vigorously  with  One  another  Under  f  Begonia,   showing  a  mass  of  crys- 

0  J  tals  composed  of  calcic  oxalate,  lying 

the    Stimulus    Of    external    forces, —  within  the  cell-cavity  around  which 

.  can  be  seen  the  living  protoplasm, 

principally      the      SUn.        Organisms  (Copied    from    a    wall-chart    by    L. 

are,  as  it  were,  little  whirlpools  in 

the  general  circulation  of  matter  and  energy.  And  I  cannot  for- 
bear to  attempt  to  illuminate  this  matter  somewhat  further  by 
aid  of  one  of  my  favorite  diagrams,  which  is  presented  herewith 
(figure  42). 

There  is  yet  one  other  group  of  substances  made  by  plants, 
very  different,  however,  in  kind  from  those  already  described. 
In  the  tissues  of  all  plants  the  microscope  reveals  mineral  matters, 
sometimes  in  great  abundance  and  crystallized  hi  very  beautiful 
forms,  of  which  our  illustration  (figure  41)  gives  some,  though  an 
inadequate  idea.  A  few  are  probably  useless  minerals  absorbed  by 
the  roots  along  with  the  useful  kinds  presently  to  be  noted,  but  the 
great  majority  are  by-products  of  useful  chemical  reactions.  Thus, 
the  commonest  of  the  crystals  is  oxalate  of  lime,  which  is  formed 
from  oxalic  acid,  probably  a  by-product  hi  the  manufacture  of 
proteins.  These  crystalline  matters  are  obviously  of  no  use,  but 
are  waste  materials.  In  the  absence  of  a  regular  excretory  system 
such  as  animals  possess,  the  plant  has  no  resource  except  to  store 


FIG.  42. — A  diagram  illustrative  of  the  relation  of  plant  and  animal  life  to  the  circulation 
of  the  principal  substances  of  nature. 


134 


The  Various  Substances  Made  by  Plants  135 


them  up  in  out  of  the  way  places,  though  they  may  ultimately 
be  partially  removed  by  the  fall  of  the  leaves  and  the  bark. 

There  remains  one  other  important  phase  of  our  subject.  It 
concerns  the  indispensability  of  certain  elements  for  healthy 
metabolism,  although  they  do  not  enter  into  the  composition 
of  any  of  the  substance  manufactured.  Everybody  knows  that 


Lacks,— all 


potas- 
sium 


iron     nothing 


nitro-      phos-      mag- 
gen       phorus  nesium 

FIG.  43.— Illustration  of  the  method  and  results  of  water  culture.    The  plants  are  Corn, 
all  started  at  the  same  time.    (Copied  from  a  wall-chart  by  En-era  and  Laurent.) 

potash  (potassium)  is  thus  indispensable,  to  such  a  degree  that 
it  must  often  be  added  as  a  fertilizer  to  soils;  but  its  symbol  (K) 
is  not  found  in  any  of  the  formulae  cited  hi  this  chapter.  The  same 
is  true  of  the  elements  calcium,  magnesium  and  iron,  and  probably 
sodium  and  chlorine,  all  of  which  are  indispensable  to  the  healthy 
growth  of  most  or  all  plants,  but  none  of  which  enter  into  the 
composition  of  the  most  important  plant  substances.  Naturally  a 


136 


The  Living  Plant 


great  many  attempts  have  been  made  to  determine  the  exact 
function  of  each  substance,  and  why  it  is  essential.  The  reader 
will  be  interested  in  the  principal  method  used  to  this  end.  It 
depends  on  the  fact  that  there  are  plants,  and  many,  which  will 

grow  through  their  whole 
cycle  from  seed  to  seed  in 
water,  without  any  contact 
with  soil,  if  only  the  needful 
minerals  be  contained  in  the 
water.  This  method  is  called 
water  culture,  and  the  prac- 
tical arrangements  therefor 
are  well  shown  in  the  accom- 
panying figure  (figure  43), 
while  a  product  of  the 
method,  produced  in  my 
own  laboratory,  is  shown  by 
figure  44.  Now,  by  growing 
one  plant  in  water  contain- 
ing all  of  the  necessary  min- 
erals except  one,  side  by  side 
with  another  plant  grown  in 
water  containing  all  of  the 

FIG.  44.-Corn  plants  growing  by  water  culture    needful  minerals,  it  IS 


in  cecn0ti^t°enr8tumbler'    The  acreen  ia  ™led  ble  to  observe   what  effect 

the  absence  of  this  one  sub- 

stance produces,  and  hence  to  infer  what  its  use  to  the  plant 
must  be.  The  general  results  of  an  experiment  of  this  kind 
are  well  shown  in  figure  43.  In  this  way  we  have  found  that 
potassium  is  necessary  to  the  formation  of  the  photosynthate, 
calcium  to  its  transfer  through  the  plant,  and  iron  to  the 
formation  of  chlorophyll  (into  the  composition  of  which,  how- 
ever, it  possibly  enters);  but  further  than  this,  and  as  to  the 
other  materials,  our  knowledge  is  most  vague  and  unsatisfac- 


The  Various  Substances  Made  by  Plants  137 

tory.  It  seems  quite  plain,  however,  that  the  role  of  these 
elements  lies  in  services  incidentally  necessary  to  the  greater 
processes, — such  as  aiding  in  chemical  steps,  neutralizing  poison- 
ous excretions,  and  so  forth.  They  are  like  the  servants  at  a 
party;  they  are  indispensable  to  its  success,  but  their  names  do 
not  appear  in  the  list  of  those  present.  But  our  ignorance  on 
these  matters,  and  upon  so  many  other  phases  of  our  subject  of 
metabolism,  is  only  acting  as  a  spur  to  the  efforts  of  many  de- 
voted workers,  who,  in  laboratories  all  over  the  world,  are  attack- 
ing these  problems  with  the  full  determination  to  solve  them. 
The  methods  of  science  are  slow,  but  they  are  irresistible;  and 
the  solution  of  the  problems  is  only  a  matter  of  time. 


CHAPTER  VI 

THE  SUBSTANCE  WHICH  IS  ALIVE  IN  PLANTS,  AND  ITS 
MANY    REMARKABLE    QUALITIES 

Protoplasm 

LREADY  more  than  once  in  this  book  the  reader  has 
met  with  a  mention  of  protoplasm, — the  living  sub- 
stance of  plants.  Besides,  almost  everyone  has  some 
knowledge  about  it,  or  thinks  that  he  has,  though  much 
of  the  current  information  is  a  very  long  way  from  the  truth. 
There  are  even  some  persons  who  believe  that  protoplasm  is  an  ab- 
stract conception  evolved  by  the  mind  of  man  to  help  explain  phe- 
nomena otherwise  incomprehensible;  while  a  few  seem  to  cherish 
the  idea  that  it  is  one  of  the  many  inventions  sought  out  by  science 
for  undermining  the  faith.  Yet  protoplasm  is  not  any  of  these 
notions,  but  a  real  material  which  can  be  seen,  handled,  and  sub- 
jected to  experiment.  The  reader  will  wish  to  know  the  facts 
about  this  most  important  of  substances,  and  here  is  the  suitable 
place  to  consider  them. 

It  is  nowadays  an  educational  axiom  that  a  good  understanding 
of  any  scientific  subject  is  possible  only  through  personal  contact 
and  experience  with  the  matter  in  question.  A  great  many  people 
do  not  comprehend  this  necessity,  and  believe  that  well-written 
and  fully-illustrated  books  are  a  sufficient,  if  not  actually  a  supe- 
rior, substitute  for  the  laborious  and  time-consuming  methods 
of  the  field  or  the  laboratory.  When  the  reader  meets  with  this 
error  he  can  refute  it  effectually  by  asking  the  objector  whether 
he  considers  that  guide-books,  even  the  best  written  and  most 

138 


The  Substance  Which  Is  Alive  in  Plants  139 

profusely  illustrated,  are  a  satisfactory  substitute  for  foreign 
travel.  The  case  is  still  stronger  with  scientific  facts  and  phenom- 
ena, for  these  are  mostly  of  a  sort  even  more  foreign  to  the  stu- 
dent's previous  experience  than  are  the  sights  and  impressions 
of  distant  lands.  All  this  is  quite  true  of  the  subject  before  us, 
and  if  the  reader  would  really  understand  the  substance  Proto- 
plasm he  must  take  steps  to  see  it  for  himself,  even  if  he  has  to 
trouble  some  friend,  his  physician,  or  the  nearest  botanical  ex- 
pert, for  the  use  of  a  microscope. 


/I       : 


I 

FIG.  45. — Typical  cells,  in  optical  section  highly  magnified,  of  hairs  from  Spiderwort, 
Gloxinia,  and  Squash,  respectively,  showing  as  accurately  as  the  author  can  represent 
it  by  pencil,  the  appearance  of  their  gray-granular  threads  and  lining  of  living  pro- 
toplasm. 

If,  now,  the  reader  will  carefully  remove  some  of  the  younger 
of  the  hairs  which  are  so  prominent  in  the  flowers  of  the  common 
Spiderwort  of  the  gardens,  (or  the  closely-related  Wandering  Jew 
of  greenhouses),  or  some  of  the  hairs  on  the  young  leaves  or  stems 
of  Squash,  or  Gloxinia,  (or  even  of  " Geranium"),  will  place  them 
on  a  glass  slide  in  a  drop  of  water,  cover  them  with  a  thin  glass, 
and  then  examine  them  with  the  microscope,  he  will  see  before  him 
living  protoplasm,  the  most  remarkable  of  all  natural  substances. 
These  particular  objects  display  an  appearance  represented  in  the 
accompanying  pictures,  (figure  45) ;  and  they  have  an  advantage 


140  The  Living  Plant 

over  others  which  might  be  chosen  in  this,  that  while  compara- 
tively easy  to  obtain,  their  protoplasm  exhibits  a  streaming 
motion,  which,  though  often  slow  and  difficult  at  first  to  detect, 
nevertheless  when  seen  forms  a  valuable  proof  of  its  living  condi- 
tion. The  rather  inconspicuous  grayish-granular,  translucent, 
semi-fluid  appearance  here  presented  inside  of  the  cells  is  repre- 
sentative of  the  aspect  of  protoplasm  in  general.  The  granular 
look  is  due  largely  to  the  presence  of  food  granules,  which  in 
some  cases  are  absent,  leaving  the  protoplasm  so  nearly  trans- 
parent that  it  can  hardly  be  seen  at  all  unless  stained  by  some  dye, 
while  in  other  cases  the  granules  are  so  plenty  as  to  give  the  proto- 
plasm an  appearance  of  solidity.  Moreover,  as  these  granules 
consist  largely  of  protein  which  has  a  slight  yellowish  color,  they 
give  to  protoplasm  in  dense  masses  a  distinctly  yellowish  or 
brownish-yellow  tinge;  and  this  is  the  cause  of  the  yellow  color 
which  shows  so  plainly  through  the  tips  of  white  roots,  and  of 
the  brownish-yellow  of  the  interior  of  young  ovules.  In  the  hairs 
supposed  to  be  lying  before  the  reader,  the  protoplasm  is  obviously 
soft  enough  to  flow  freely,  though  it  is  not  wholly  a  fluid;  and  it 
is  known  to  possess  about  the  consistency  of  a  soft  jelly.  Indeed, 
if  one  were  to  imagine  an  uncolored  jelly,  somewhat  too  soft  to 
retain  the  form  of  its  mold  and  all  clouded  instead  of  quite  clear, 
— hi  other  words  just  the  kind  of  jelly  that  the  thrifty  house- 
keeper doth  most  despise,  he  would  have  a  very  good  idea  of  the 
protoplasm  of  these  hairs.  In  some  plant  tissues  the  substance 
is  still  softer  and  almost  a  liquid;  in  others  it  is  firmer,  to  such  a 
degree  that  in  seeds  it  becomes  tough  and  hard  as  horn,  though 
never  approaching  the  hardness  of  ivory,  as  a  prominent  diction- 
ary says  that  it  does.  The  visible  streaming  of  the  protoplasm 
in  these  hairs,  however,  is  not  typical,  for  while  in  some  kinds 
the  streaming  is  even  more  active,  generally  it  is  very  much  slower, 
and  commonly  is  imperceptible;  so  that  the  reader  must  not 
allow  the  motion  to  become  too  prominent  a  feature  of  his  vis- 
ualization of  plant  protoplasm.  A  white-granular,  slow-moving 


The  Substance  Which  Is  Alive  in  Plants  141 

jelly; — that  is  what  protoplasm  looks  like,  and  that  is  precisely 
what  it  is.* 

While  protoplasm  for  the  most  part  can  be  observed  in  plants 
only  by  aid  of  the  microscope,  there  are  cases  in  which  it  occurs 
in  masses  sufficiently  large  to  be  studied  by  the  unaided  eye,  and 
to  be  taken  in  the  hand.  Everybody  has  seen  those  soft,  whitish, 
slimy  masses  which  are  flattened  against  decaying  wood  in  damp 
dark  places,  such  as  the  rotten  underpinning  of  old  buildings, 
in  cellars  and  dark  greenhouses,  or  on  old  shaded  tan-bark, — 
whence  they  are  known  as  " Flowers  of  Tan."  These  are  called, 
scientifically,  Slime-molds,  and  they  are  practically  pure  naked 
protoplasm,  the  accessibility  of  which  has  made  these  low  plants 
very  favorite  objects  for  protoplasmic  studies. 

Such  is  the  appearance  of  living  plant  protoplasm  as  seen 
by  the  eye  or  through  an  ordinary  microscope;  and  try  as  one 
will,  he  can  see  little  more.  The  supreme  importance  of  proto- 
plasm among  earthly  substances  has  of  course  acted  as  a  stimulus 
to  the  most  thorough  researches  into  its  structure;  and  all  the 
highest  powers  of  the  microscope,  and  all  the  most  refined  de- 
vices and  methods  known  to  microscopical  science,  have  been 
brought  to  bear  upon  it.  Yet  these  efforts  have  yielded  little 
additional  knowledge,  and  even  that  little  has  been  left  involved 
in  uncertainty  and  controversy.  We  do  not  even  know  what  tex- 
ture the  protoplasmic  substance  possesses.  Some  investigators 
have  concluded  that  such  protoplasm  as  the  reader  has  seen 
streaming  in  plant-hairs  is  a  loose  network  of  fine  elastic  fibers, 

*  The  streaming  of  Protoplasm  is  thus  vividly  visualized,  though  with  some  ex- 
aggeration natural  at  that  time,  by  Huxley, — "Currents  similar  to  those  of  the  hairs 
of  the  nettle  have  been  observed  in  a  great  multitude  of  very  different  plants,  and 
weighty  authorities  have  suggested  that  they  probably  occur,  in  more  or  less  per- 
fection, in  all  young  vegetable  cells.  If  such  be  the  case,  the  wonderful  noonday 
silence  of  a  tropical  forest  is,  after  all,  due  only  to  the  dulness  of  our  hearing;  and 
could  our  ears  catch  the  murmur  of  these  tiny  Maelstroms,  as  they  whirl  in  the  in- 
numerable myriads  of  living  cells  which  constitute  each  tree,  we  should  be  stunned, 
as  with  the  roar  of  a  great  city."  The  Physical  Basis  of  Life  in  his  Collected  Essays, 
New  York,  I,  136. 


142  The  Living  Plant 

holding  liquids  in  its  meshes  as  a  sponge  might  do, — a  view  more 
prevalent  formerly  than  now,  even  though  it  is  sustained  by  the 
appearance  of  the  substance  when  killed  and  colored  by  dyes. 
Others  consider  that  protoplasm,  aside  from  certain  solid  gran- 
ules, is  chiefly  an  emulsion  of  various  liquids,  which  rest  suspended 
as  tiny  globes  in  a  matrix  of  fluid  ground  substance,  very  much  as 
the  tiny  globules  of  oils  remain  suspended  in  water  after  violent 
shaking  of  a  mixture.  And  the  advocates  of  this  view,  now  in  the 
ascendant,  have  supported  it  by  constructing,  out  of  ordinary 
chemicals,  certain  emulsions  or  foams,  which  show  striking  sim- 
ilarities to  living  protoplasm  not  only  in  appearance,  but  in  move- 
ments, though  they  are,  however,  far  enough  removed  from 
protoplasm  in  all  other  respects.  And  a  third  view  tries  to  har- 
monize the  two  others  by  supposing  that  some  protoplasm  has 
one  structure  and  some  the  other.  In  one  part  only  does  proto- 
plasm display  a  definite  structure,  and  that  is  in  the  nucleus  dur- 
ing reproduction,  a  matter  we  shall  presently  consider. 

It  may  seem  to  the  reader  remarkable  that  I  do  not  attempt  to 
illustrate  so  important  a  subject  more  fully  by  pictures.  But 
protoplasm  in  fact,  because  of  the  lack  of  clear  definition  in  its 
structure,  is  most  difficult  to  represent  well  in  any  kind  of  pic- 
ture. Indeed,  hardly  any  two  persons  represent  it  alike,  as 
follows  naturally  enough  from  the  fact  that  hardly  any  two  per- 
sons see  it  alike.  In  various  figures  in  this  book,  however,  I  have 
tried  incidentally  to  give  some,  even  though  rather  a  conventional, 
idea  of  its  appearance,  and  to  these  figures  (figures  33,  34,  39,  40, 
41,  45)  the  reader  will  now  find  it  worth  while  to  refer.  And  I  shall 
at  this  point,  add  one  more,  and  one  of  the  best,  in  which  the  great 
botanist  Sachs  has  tried  to  represent  it  as  if  projected  against 
a  black  background,  (figure  46). 

We  come  now  to  the  important  matter  of  the  chemical  com- 
position of  protoplasm,  from  which,  in  view  of  its  many  remark- 
able powers,  we  naturally  anticipate  something  of  very  unusual 
interest.  The  most  striking  of  the  chemical  facts  about  it,  as  the 


The  Substance  Which  Is  Alive  in  Plants  143 

chapter  on  Metabolism  further  illustrates,  is  this, — that  proto- 
plasm, despite  its  aspect  of  simplicity,  is  not  a  single  substance, 
but  a  very  heterogeneous  mixture  of  many  different  substances 
of  diverse  grades  of  complexity,  from  the  simplest  of  mineral 
salts  up  to  the  most  complicated  of  pro- 
teins. None  of  these  substances,  how- 
ever, are  of  themselves  alive,  nor  has 
chemical  analysis  yet  succeeded  in  lo- 
cating any  distinctively  living  constit- 
uent,— any  protoplasm  par  excellence, 
although  we  are  logically  bound  to  be- 
lieve that  some  such  substance  must 
exist  as  a  seat  for  the  distinctive  prop- 
erties of  life.  Protoplasm,  therefore,  is 
probably  composed  chemically  of  two 
classes  of  materials; — first,  a  very  small 
amount  of  a  distinctively  living  constit- 
uent, not  yet  identified,  but  consisting, 
in  the  fibers,  or  else  the  ground  substance 
of  its  physical  texture ;  and  second,  a  very 
large  amount  of  various  non-living  sub- 
stances, nutritive  and  other,  which  are 
under  the  control  of  the  living  constit- 
uent. 

There    are,    however,    some    further 
chemical  facts  about  protoplasm  which   FlG.  46._The  protoplasm  of  a 
go  a  little  way  towards  explaining  its      hair  ceil  of  a  Gourd,  projected 

against  a  black  background. 
Various  powers.     Thus,  a  part  Of  its  COn-         (Reduced    from   Sachs'    Lec- 

stituents  (in  general  the  most  compli- 
cated) are  very  unstable,  or,  chemically  stated,  labile,  and 
change  their  composition  under  slight  provocation  whether  from 
without  or  within.  Such  changes  are  accompanied,  like  all 
others  of  a  chemical  nature,  by  transformations  of  energy,  either 
release  or  absorption.  And  these  in  turn  cause  other  changes, 


144  The  Living  Plant 

and  these  yet  others,  in  an  almost  endless  succession.  Thus  liv- 
ing protoplasm,  complex  and  unstable  in  its  constituents,  and 
acted  upon  constantly  by  diverse  forces  both  from  without  and 
within,  is  a  constantly  seething  mass  of  energy-and-material 
changes; — and  it  is  such  changes  which  constitute  the  visible 
phenomena  of  life.  But, — and  here  is  the  crux  of  the  matter, — 
these  changes  are  not  hap-hazard  and  aimless,  but  on  the  contrary 
proceed  hi  a  definite  and  orderly  sequence,  resulting  in  the  forma- 
tion of  definite  structures  and  the  performance  of  definite  actions 
time  after  time  and  generation  after  generation;  and  it  is  this 
orderliness,  this  definite  procession  of  physical  and  chemical 
processes,  rather  than  anything  in  the  processes  themselves, 
which  is  the  most  distinctive  characteristic  of  life.  The  failure 
of  the  regulatory  power  breaks  the  circuit  of  the  processes,  and 
leaves  the  protoplasm  a  helpless  mass  of  matter  all  ready  for  de- 
cay; and  this  failure  we  name  death.  Life  thus  consists  of  two 
elements,  first,  material  and  energy  changes,  that  is,  purely  physi- 
cal and  chemical  processes,  whose  general  nature  we  can  under- 
stand, and  which  are  seated  in  the  various  substances  that  chem- 
ists have  identified  in  the  protoplasm,  and  second,  a  regulatory 
power  which  directs  and  makes  use  of  those  processes  but  whose 
nature  and  location  is  still  quite  unknown.  Perhaps  the  nature 
of  this  regulatory  power  is  incomprehensible,  or  unknowable,  in 
our  present  philosophies,  though  as  to  that,  science  never  admits 
that  anything  is  unknowable,  but  works  ever  under  the  assump- 
tion that  everything  can  be  known  if  we  but  refine  sufficiently  our 
methods  of  investigation. 

There  is  one  other  feature  of  the  chemistry  of  protoplasm 
which  may  have  some  importance  in  explaining  its  powers.  In  a 
general  way  it  seems  true  that  the  protoplasm  of  the  higher  and 
more  elaborate  plants  and  animals  is  more  complicated  chemi- 
cally, or  at  all  events  produces  a  greater  number  of  complicated 
substances  (proteins  especially),  than  the  lower.  This  suggests 
that  each  of  the  special  physiological  features  successively  ac- 


The  Substance  Which  Is  Alive  in  Plants  145 

quired  by  plants  and  animals  in  the  course  of  their  evolution 
has  its  seat  in  a  special  chemical  constituent  of  the  protoplasm. 
On  this  view,  evolution,  physiologically  considered,  depends  upon 
chemical  experimentation,  so  to  speak,  in  the  protoplasm,  and 
follows  step  by  step  on  the  successful  formation  of  new  chemical 
compounds.  But  let  the  reader  beware  of  accepting  this  sug- 
gestion as  knowledge;  it  is  merely  a  speculation,  but  one  of  those 
which,  in  science,  it  is  legitimate  to  throw  out  ahead  as  a  tem- 
porary guide  to  further  investigation. 

In  common  with  all  other  substances  in  Nature,  protoplasm 
thus  possesses  its  physical  and  chemical  properties.  But  in  ad- 
dition it  possesses  another  set  not  found  in  other  substance; 
and  thereupon  depend  its  powers  to  do  the  remarkable  things 
that  it  does.  These  may  be  termed  its  physiological  or  vital 
properties,  which  are  as  follows; — the  property  of  metabolism,  or 
power  of  causing  orderly  chemical  changes  within  itself,  including 
photosynthesis  and  respiration,  and  the  other  changes  recorded 
in  our  chapter  devoted  to  that  particular  subject:  the  property 
of  conduction,  or  power  to  transport  substances  in  definite  paths 
through  itself,  including  absorption,  transfer,  and  excretion:  the 
property  of  growth,  or  power  to  incorporate  new  material  and  to 
increase  in  size  at  special  places :  the  property  of  division,  or  power 
to  separate  portions  of  its  own  substance,  the  basis  of  reproduc- 
tion: the  property  of  mobility,  or  power  to  cause  definite  move- 
ments of  its  own  substance,  the  basis  of  protoplasmic  streaming 
and  locomotion:  the  property  of  irritability  (sensitivity),  or  power 
to  respond  advantageously  to  various  stimuli.  This  enumeration 
of  the  physiological  properties  of  protoplasm  reads  like  the  table 
of  contents  of  a  book  on  physiology, — and  it  ought  to,  because 
physiology  is  nothing  else  than  a  study  of  the  properties  of  proto- 
plasm. And  here  is  a  point  of  importance.  Just  as  the  physical 
properties  of  any  substance  are  believed  to  reside  in  certain  ulti- 
mate structural  units,  which  are  the  smallest  portions  into  which 
that  substance  can  be  divided  and  still  retain  those  properties, 


146  The  Living  Plant 

and  which  units  in  this  case  are  the  molecules,  and  just  as  the 
chemical  properties  are  supposed  to  reside  in  their  ultimate  units, 
in  this  case  the  atoms, — so  the  vital  properties  must  be  supposed 
to  reside  in  some  kind  of  units  distinctively  their  own.  These 
units,  obviously,  must  be  larger  than  the  molecules  and  made  up 
of  organized  aggregates  thereof.  They  have  been  called  by  various 
names,  notably  plasomen,  (in  the  singular,  plasom),  and  are 
probably  identical  with  the  micellae  of  which  we  shall  have  much 
to  say  in  the  chapter  on  Absorption.  All  substances  are  made  up 
of  atoms  and  molecules;  protoplasm  alone  is  made  up  of  atoms, 
molecules  and  plasomen.  And  the  reader  will  observe,  by  the 
way,  that  the  very  conception  of  the  plasom  involves  the  idea  of  a 
distinctive  protoplasmic  main  substance,  and  constitutes  indeed, 
an  additional  reason  for  believing  in  the  existence  thereof. 

As  one  views  the  various  physical  features  of  protoplasm,  and 
thinks  of  the  remarkable  things  it  can  do,  he  cannot  but  wonder 
at  the  discrepancy  between  its  aspect  and  its  accomplishments. 
For  protoplasm  is  one  of  the  most  insignificant  in  appearance 
of  all  substances,  yet  secures  the  most  wonderful  of  all  results. 
For  has  it  not  built  the  whole  plant  and  animal  world,  culminating 
in  man  with  his  powers  of  thought?  Yet  this  discrepancy  be- 
tween promise  and  performance  is  not  without  parallel  in  our 
human  experience.  If  some  stranger  from  far  away  space,  where 
all  things  are  differently  done,  were  to  visit  this  earth  and  be 
shown  the  multifarious  works  of  man's  hands,  and  were  after- 
wards to  have  man  pointed  out  as  their  maker,  he  would  doubtless 
exclaim  in  astonishment; — "How  can  a  creature  so  small  build 
these  cloud-cleaving  towers  a  hundred  times  loftier  than  himself, 
or  these  huge  leviathans  of  steamships  ten  thousand  times  bigger 
than  he :  or  how  can  a  thing  so  weak  raise  pyramids  so  ponderously 
colossal:  or  one  so  slow  of  foot  drive  such  fleet-flying  engines: 
or  one  with  hands  so  soft  bore  tunnels  through  miles  of  solid 
rock?"  Man  gives  no  suggestion  in  his  appearance  of  the  nature 
of  the  power  whereby  he  does  these  things,  for  that  lies  not  in 


The  Substance  Which  Is  Alive  in  Plants  147 

his  visible  body  but  his  invisible  mind,  which  enables  him  to 
plan  and  make  use  of  tools,  and  harness  the  restless  forces  of 
nature.  So,  we  can  only  suppose  that  the  physically-insignificant 
protoplasm  accomplishes  its  results  by  some  analogous  power. 
Indeed,  I  venture  for  my  part  to  believe  that  all  protoplasm  can 
think, — not  mind-thought  it  is  true,  for  that  appears  to  belong 
only  to  man,  but  body-thought  of  which  the  mind  is  unconscious. 
Or  the  matter  may  better  be  stated  in  this  way,  that  man's  thought 
is  but  the  conscious  form  of  a  principle  which  exists  unconsciously 
through  all  living  substance.  All  protoplasm  thinks,  but  only 
the  portion  thereof  in  man's  brain  is  aware  that  it  thinks.  How- 
ever this  may  be,  there  is  one  thing  that  is  plain; — man's  is  not 
the  only  protoplasm  which  makes  use  of  tools,  and  compels  the 
forces  of  nature  to  do  its  work,  in  evidence  whereof  let  the  reader 
observe,  for  example,  what  is  said  in  this  book  about  enzymes, 
and  the  dissemination  of  seeds. 

We  must  here  turn  back  for  a  moment  to  the  chemistry  of 
protoplasm  in  order  to  notice  a  matter  important  to  an  under- 
standing of  the  relations  of  the  substance  to  the  external  world. 
The  chemical  complexity  and  instability  of  protoplasm  render 
it  extremely  sensitive  to  the  effects  of  external  influences,  which 
act  upon  it  in  three  different  ways.  First,  if  strong  enough,  they 
act  upon  it  forcibly,  precisely  as  upon  any  other  substance  of 
comparable  sort,  and  quite  without  reference  to  whether  it  is 
living  or  not.  Thus,  heat  burns  it;  pressure  crushes  it;  and  some 
chemicals  dissolve  it.  Second,  the  forces  when  too  weak  to  exert 
any  forcible  effects,  can  yet  act  inductively  to  promote,  or  to 
check,  some  of  the  processes  in  progress  in  the  complicated  chemi- 
cal laboratory  which  the  living  protoplasm  actually  is,  and  thereby 
may  produce  a  profound  effect  upon  the  behavior  of  the  plant 
as  a  whole.  Thus  heat,  in  a  degree  far  too  low  to  injure  the 
protoplasm,  promotes  the  activity  of  those  physical  and  chemical 
reactions  which  underlie  the  streaming,  nutrition,  growth  and 
other  activities  of  protoplasm;  and  this  explains  why  protoplasm 


i48  The  Living  Plant 

streams  faster,  and  plants  grow  better,  in  warmth  than  in  cold. 
Light  acts  analogously  on  the  cell-contents,  and  one  of  the  results 
is  the  brilliant  redness  of  autumn  coloration.  In  some  cases  the 
external  factors,  especially  some  chemical  substances,  act  repress- 
ively  on  the  processes,  which  explains  the  action  of  anaesthetics. 
Third,  the  factors,  when  far  too  weak  to  exert  even  an  inductive 
effect  can  act  in  a  far  more  remarkable  and  consequential  manner, 
for  they  can  then  serve  as  guides,  or  stimuli,  in  response  te  which 
the  protoplasm  can  send  its  parts  into  positions  found  by  past 
experience  to  be  best  for  the  performance  of  its  functions  or  avoid- 
ance of  dangers.  Thus,  light  far  too  weak  to  be  directly  useful 
or  injurious  to  the  plant  yet  serves  as  a  guide  whereby  stems  can 
grow  towards  it,  leaves  across  it,  and  roots  away  from  it,  those 
positions  being  the  most  advantageous  for  the  performance  of 
their  particular  functions.  And  innumerable  other  cases  of  this 
kind  are  known,  of  such  interest  and  importance,  however,  that 
they  must  receive  a  chapter  all  to  themselves  under  their  proper 
physiological  name  of  Irritability.  It  is  enough  for  our  purpose 
at  present  to  make  clear  the  existence  of  the  three-kind  relation 
between  protoplasmic  activity  and  the  external  world. 

One  does  not  go  far  with  his  studies  upon  protoplasm  before 
he  begins  to  take  thought  of  its  origin.  In  one  way  the  prob- 
lem is  simple  enough,  for  all  of  the  protoplasm  familiar  to  us 
originates  obviously  in  only  one  way, — by  growth  and  division 
from  other  protoplasm  through  reproduction.  It  is  not  so  long 
since  even  scientific  men  held  the  contrary  belief,  still  widely 
persistent  among  uneducated  folk,  that  low  forms  of  life  could 
originate  anew  in  slime  or  other  fermentable  masses;  but  later 
experimental  studies,  chiefly  led  by  the  great  Frenchman  Pasteur, 
have  shown  that  in  all  such  cases  living  germs  are  present,  while 
if  precautions  are  taken  to  kill  all  germs  by  heat  or  suitable 
poisons,  then  no  life  appears.  Every  known  case  of  apparent 
spontaneous  generation  having  thus  been  investigated  and  dis- 
proved, we  infer  that  probably  it  does  not  now  occur  in  our 


The  Substance  Which  Is  Alive  in  Plants  149 

world,  and  that  all  protoplasm  nowadays  originates  from  pre- 
existent  protoplasm  through  reproduction.  This  much  is  easy. 
But  when  we  try  to  trace  back  the  continuously-reproducing 
chain  to  its  very  first  origin  in  time,  we  come  soon  to  the  limits 
of  our  knowledge.  Some  philosophers  have  suggested  that  the 
germs  of  life  were  first  brought  to  the  earth  in  meteorites  from 
other  planets;  but  this  merely  sets  back  the  difficulty  one  stage 
and  does  not  remove  it.  Another  explanation,  which  seems  to  be 
that  most  commonly  assumed  by  scientific  men,  places  its  origin 
in  spontaneous  generation  at  some  time  in  the  earth's  history 
when  the  favorable  combination  of  material  and  energy  happened 
to  occur.  Obviously,  such  a  combination  ought  to  be  repeatable 
experimentally;  and  it  is  upon  this  assumption  that  many  learned 
men,  from  astrologers  of  old  to  physiologists  now  with  us,  have 
sought,  though  in  vain,  to  make  protoplasm  anew  in  the  flasks 
of  their  laboratories.  There  is,  however,  a  third  explanation  which 
I  have  already  suggested  in  an  earlier  chapter, — namely,  that  the 
protoplasm  known  to  us  did  not  originate  in  its  present  form, 
but  is  evolved  or  descended  from  a  simpler  substance  adapted 
chemically  to  the  higher  (or  lower)  temperatures  which  formerly 
prevailed  on  the  earth,  while  that  substance  in  turn  was  evolved 
from  a  still  simpler,  and  so  on  backwards  to  a  beginning  cotempo- 
raneous  with  that  of  inorganic  matter  itself.  This  view  I  hold 
to  be  the  most  reasonable  and  probable. 

But,  after  all,  the  most  impressive  and  important  thing  about 
protoplasm  is  its  power  to  build  those  great  and  elaborate  struc- 
tures which  we  call  plants  and  animals.  For,  structurally  con- 
sidered, a  plant  or  an  animal  is  nothing  other  than  a  mass  of 
soft  protoplasm  which  climbs  aloft  and  reaches  outward  into  the 
form  of  the  plant  or  the  animal,  building  itself  meantime  a  skele- 
ton for  the  support  of  its  helplessly-weak  substance.  Now,  in 
building  these  organisms,  the  protoplasm  never  exhibits  the  char- 
acter of  a  continuous  and  homogeneous  mass,  but  always  sepa- 
rates partially  into  tiny  structural  units  called  cells,  which  are 


150  The  Living  Plant 

mostly  too  small  to  be  seen  by  the  naked  eye,  but  which  appear 
prominently  in  every  magnified  view  of  any  part  of  any  animal 
or  plant,  as  witness,  for  example,  figures  2,  53,  73, 141,  in  this  book. 
We  must  therefore  consider  with  some  care  the  construction  of 
these  cells, — a  subject  of  the  foremost  importance  in  Biology. 
The  hairs  earlier  studied  are  fairly  typical  cells  except  that 
they  are  partially  isolated  from  their 
neighbors  instead  of  deeply  em- 
bedded among  them,  and  are  elon- 
gated rather  than  rounded.  If  one 
observes  an  example  of  these  hairs, 
e.  g.,  that  of  the  Squash  (figure  47), 
he  is  likely  to  notice  first  the  clear- 
cut  containing  wall,  inside  of  which 
sap-cavity  comes  a  complete  lining  of  soft  gray- 
granular  protoplasm,  very  likely  in 
slow  streaming  motion,  with  threads 
of  the  same  extending  across  the  cell 


Squash,  showing  all  the  parts  of  a    at  VaHOUS   angles.       This    Soft 

plasm  is  called  cytoplasm.  Some- 
where within  it,  though  not  carried  in  its  streaming,  lies  a  denser, 
rounded  granular  structure,  also  living  protoplasm,  the  nucleus, 
which  often  exhibits  a  round  mass  within  itself, — the  nudeolus. 
In  the  cytoplasm  lie  also  certain  scattered  granules  (not  especially 
distinct,  however,  in  these  hairs),  which  are  larger  than  food  gran- 
ules and  otherwise  unlike  them;  these  too,  are  living  protoplasm, 
and  are  called  plastids.  Finally,  within  the  cytoplasm  appear 
large  open  spaces,  various  in  size  and  number  but  commonly 
merged  to  a  single  very  large  one  in  old  cells;  though  apparently 
empty,  they  really  are  filled  with  a  watery  sap  and  therefore  are 
known  as  sap-cavities.  These  parts,  wall,  cytoplasm,  nucleus, 
plastids,  sap-cavities,  are  the  prominent  parts  of  typical  plant 
cells,  and  the  great  majority  of  cells  possess  them  all.  We  can 
accordingly  construct  a  conventionalized  cell  showing  these 


The  Substance  Which  Is  Alive  in  Plants  151 

parts  in  their  natural  relations  and  fully-developed  condition; 
and  such  a  cell  is  represented  herewith  (figure  48) . 

We  should  now  examine  a  bit  further  these  parts  of  the  cell 
and  their  meaning. 

The  wall  is  composed  of  a  firm-elastic  transparent  substance 
called  cellulose,  whose  chemistry  is  treated  in  the  chapter  on 
Metabolism.  It  is  built  by  the  cytoplasm,  which,  in  suitable 
places,  is  supposed  to  lay  down  within  itself  tiny  masses  (bricks, 
as  it  were)  called  micella?,  of  cellulose,  and  continues  to  add  to 


FIG.  48. — An  optical  section  through  a  conventionalized  complete  plant  cell. 

their  number  until  they  accumulate  to  nearly  a  solid  mass.  I 
say  " nearly,"  because  apparently  there  always  are  left  between 
these  micella?  thin  sheets  of  protoplasm,  like  the  mortar  between 
bricks,  so  long  as  the  cells  are  alive,  though  they  are  withdrawn 
when  the  cell  has  reached  full  maturity.  It  is  these  thin  sheets 
of  cytoplasm,  too  thin  to  be  visible  even  to  the  strongest  micro- 
scope, which  keep  the  wall  alive,  as  it  were,  so  that  it  can  become 
enlarged,  split,  chemically  changed,  absorbed  in  places,  and  in 
other  ways  altered,  a  good  while  after  its  formation.  But  except 
for  such  subsequent  alterations,  the  walls  of  contiguous  cells  re- 


152  The  Living  Plant 

main  parts  of  one  continuous  mass.  As  to  the  function  of  the  wall, 
that  is  perfectly  obvious, — it  is  the  skeleton  of  the  cell,  the  me- 
chanical support  for  the  gelatinous  cytoplasm,  which  has  not 
enough  firmness  of  texture  to  raise  itself  unaided  an  inch  from  the 
ground.  It  is  interesting  to  let  the  imagination  picture  what 
would  happen  to  the  loftiest  and  stateliest  tree,  if,  by  some  subtle 
chemical  magic  the  cell- walls  could  be  suddenly  re-converted  back 
to  the  gases  from  which  they  were  made;  the  protoplasm  would 
simply  collapse  to  the  ground  as  a  shower  of  slime. 

The  reader  at  this  point  will  observe  how  different  in  principle 
is  the  construction  of  the  skeleton  in  plants  as  compared  with 
animals.  In  animals,  in  conformity  with  the  much  higher  de- 
gree of  division  of  labor  in  their  parts,  certain  cells  are  set  aside 
to  build  the  skeleton  for  the  entire  individual,  either  a  deeply- 
buried  bony  skeleton  as  in  man,  or  a  surface  skeleton  of  lime 
or  horn  as  in  crabs  and  insects;  while  all  of  the  remainder  of  their 
cells  are  without  hard  walls  and  devoted  to  other  functions. 
In  plants,  however,  every  individual  cell  has  a  wall  around  it- 
self, and  the  collective  mass  of  these  walls  makes  up  the  skeleton 
of  the  plant.  Such  a  mass  of  cell-walls,  however,  by  no  means 
represents,  though  one  might  naturally  think  so,  a  lot  of  origi- 
nally separate  walls  fused  together.  Observation  of  growing  parts 
always  shows  (figure  101)  that  the  new  walls  formed  between 
dividing  cells  are  thrown  across  the  protoplasm  as  single  solid 
structures,  which  may  or  may  not  in  time  become  split  and 
divided  between  the  two  cells.  Thus  the  cell-wall  system  of  a 
plant  is  one  single  mass  from  the  beginning,  just  as  is  the  wall 
mass  of  a  building;  and  the  protoplasm  lives  in  cavities  therein, 
precisely  as  people  live  in  the  rooms  of  a  house  they  have  built. 
The  reason  for  the  difference  in  the  method  of  skeleton  building 
by  animals  and  plants  is  plain  enough  upon  reflection.  The 
method  of  animals  permits  jointing  and  muscular  movement, 
as  it  must  in  order  to  allow  the  most  fundamental  of  all  animal 
activities, — locomotion  in  search  of  food;  the  method  of  plants 


The  Substance  Which  Is  Alive  in  Plants  153 

permits  only  a  fixed  position,  which,  however,  is  sufficient,  since 
the  materials  for  making  their  food  are  brought  to  them  in  the 
general  circulation  of  nature.  And  these  conclusions  are  all  the 
more  confirmed  by  the  seeming  exceptions,  for  some  plants  swim 
or  creep  freely  about  (e.  g.,  swimming  spores  of  Algae  and  Slime 
Molds)  in  a  very  animal-like  manner;  but  in  these  cases  they  lack 
the  firm  cellulose  wall  distinctive  of  plants.  But  although  the 
skeletons  of  animals  and  plants  differ,  their  protoplasm  does  not, 
for  in  all  essentials  the  protoplasm  of  plants  and  animals  is  alike. 
This  brief  account  of  the  plant  skeleton  has  touched  incidentally 
on  a  matter  which  must  now  receive  some  further  attention.  As 
the  student  soon  learns  when  he  studies  many  cells  with  his 
microscope,  they  differ  immensely  in  shape  and  in  the  thickness 
and  composition  of  their  walls,  to  such  a  degree  indeed  as  to 
make  them  apparently  too  complex  for  analysis.  Yet  here,  as 
elsewhere,  further  study  gradually  crystallizes  out  the  essentials, 
when  it  appears  that  after  all  only  a  few  ground  forms  exist,  and 
then  only  in  correlation  with  definite  functions  or  influences; 
while  all  of  the  others  are  simply  variations  and  combinations  of 
these.  As  to  the  shapes  of  cells,  the  simplest  of  all,  and  the  one 
to  which  all  others  tend  to  revert,  is  the  sphere,  that  being  the 
mathematical  form  in  which  the  most  contents  can  be  comprised 
within  the  least  wall.  This  shape,  with  the  wall  a  spherical  shell, 
is  actually  realized  in  those  cells  which  float  freely  in  water  or 
air,  as  do  the  spores  of  many  Algae  and  Molds,  and  some  pollen 
grains;  and  this  shape  may  become  elongated  to  ellipsoid  and  ovoid 
forms  under  particular  conditions  (figure  49,  94, 108).  Where  such 
cells  occur  inside  the  tissues  of  plants,  however,  and  hence  are 
hard  pressed  by  their  numerous  neighbors,  the  spherical  shape 
becomes  necessarily  modified  to  many-sided  (polyhedral)  or 
faceted;  and  this  shape  is  approximately  realized  in  many  stor- 
age tissues  of  plants,  where  it  comes  measurably  near  to  that 
twelve-faced  shape  which  always  results  when  equal-sized  spheres 
are  forced  together  by  pressure  (figure  49,  72).  There  is  also  some 


154 


The  Living  Plant 


U 


0  o 
O 


FIG.  49. — Generalized  drawings  of  optical  sections  through  the  principal  forms  of  plant 
cells,  all  of  which  are  derivable  by  differential  growth  from  the  spherical  form  in  the 
center. 

approach  to  this  shape  in  the  green  cells  of  leaves,  (figure  2),  al- 
though here  a  modification  is  introduced  by  the  need  for  con- 
centrating the  chlorophyll  grains  towards  the  best-lighted  sur- 
face, for  which  a  cylindrical  shape  is  the  best  (Plate  I,  B),  or  else 


The  Substance  Which  Is  Alive  in  Plants  155 

by  the  need  for  the  presence  of  very  large  air  spaces,  for  which  a 
branching,  or  stellate  form  is  most  suitable.  The  polyhedral 
shape  due  to  mutual  pressures,  in  conjunction  with  the  formation 
of  new  walls  as  plates  thrown  across  cells  from  one  wall  to  the 
other,  results  in  the  formation  of  cubical  cells  in  growing  points 
(figures  49,  53,  139  CD),  or  elongations  thereof  to  four-sided 
prisms,  as  in  the  cambium  cells,  which  form  the  growth  zone 
between  the  bark  and  the  wood  in  most  trees  (figure  139  B).  In 
other  cases  the  cells  become  flattened  to  tabular  shapes,  as  in 
epidermis  and  cork  (figures  2,  49) ;  where  the  function  of  those 
cells  as  the  protective  skin  of  the  plant  obviously  requires  such  a 
shape.  Again,  the  spherical  or  polyhedral  shape  becomes  elon- 
gated to  a  cylindrical  or  prismatic  form  where  the  function  re- 
quires much  length,  as  it  does  in  the  conduction  of  liquids  through 
the  plant;  and  it  is  a  line  of  such  cylindrical  cells,  thrown  into 
a  tube  by  absorption  of  the  intermediate  walls,  which  constitutes 
the  water-carrying  ducts,  (figures  49,  53,  54 C,  72)  while  the  food- 
carrying  sieve-tubes  are  made  in  analogous  manner  (figure  72). 
Or,  the  elongation  takes  place  at  two  opposite  points,  result- 
ing in  a  spindle  or  fiber  form,  which  is  developed  wherever  tensile 
strength  for  resistance  to  strains  is  required  (figures  49,  50  d) .  Fi- 
nally, through  the  intermediation  of  a  more  active  growth  at  several 
points,  the  spherical  or  polyhedral  shape  becomes  modified  to  a 
branching,  or  even  a  star-shaped  form;  and  this  occurs  in  the 
spongy  cells  of  green  leaves  as  a  means  of  providing  generous 
inter-cellular  air  spaces,  (figure  2,  and  B  of  Plate  I) :  in  some 
Rushes  as  a  part  of  their  very  flexible  pith  (figure  49) :  and  in 
certain  excretion  cells  of  Water-plants  as  a  means  of  providing 
more  wall  for  the  deposition  of  waste  crystals.  Thus  these  few 
ground  forms, — the  fundamental  sphere,  with  its  lines  of  modifi- 
cation, shown  by  figure  49, — viz.,  ellipsoid-ovoid,  polyhedral, 
tabular,  cylindrical-tubular,  spindle-fibroid,  and  branched-stellate, 
represent  the  mathematical  possibilities  upon  which  the  cells  can 
play,  but  by  which  they  are  also  bound  in  their  adaptations  to 


156 


The  Living  Plant 


their  various  functions;  and  although  innumerable  forms  occur 
not  directly  referable  to  any  of  these  types,  they  are  never- 
theless only  modifications  and  combinations  thereof. 

The  cell-wall,  however,  is  modifiable  not  only  in  shape,  but  also 
in  thickness.  Ordinarily  very  thin,  it  can  become  thickened  to 
any  degree  required  by  function,  even  to  the  almost  total  ob- 
literation of  the  cell  cavity,  as  happens  in  some  fibers  (figure  50,  d), 
where  the  need  for  additional  strength  is  perfectly  plain :  in  cells 


*r 


FIG.  50. — Various  methods  of  adaptive  thickening  of  cell-walls;  further  particulars  in  text. 
(All  copied  from  von  Mohl's  classical  work  on  the  Plant  Cell,  1851.) 

devoted  to  the  protection  of  something,  notably  in  the  shell  of  a 
nut  (figure  50,  a) :  and  in  cases  where  the  formation  of  a  thickened 
wall  is  a  means  of  storing  additional  food,  as  in  the  Ivory  Palm  and 
Date  (figure  36).  A  similar  thickening  is  used  also  as  a  pro- 
tection to  the  resting  spores  of  Molds,  Yeasts  and  disease  germs, 
which  thereby  are  so  completely  protected  against  all  hostile 
outside  influences  that  they  can  float  uninjured  for  months  in 
the  air,  and  germinate  finally  in  the  most  unexpected  and  least 
desired  of  places.  In  some  cases  the  thickening  is  not  at  all  uni- 
form, but  takes  the  form  of  rings  and  spirals,  as  in  young  ducts 
which  they  help  to  keep  open  while  the  walls  are  still  very  flex- 
ible (figure  50,  6) ;  or  it  makes  an  elaborate  fretwork  of  strength- 
ening ridges  surrounding  thin  areas  easily  pervious  to  water, 
as  in  older  ducts  (figure  50,  c) ;  or  it  occurs  upon  one  wall  only,  as 


The  Substance  Which  Is  Alive  in  Plants  157 

is  frequently  the  case  with  protective  epidermal  cells  (figure  50,  e) ; 
or  it  affects  only  the  angles,  in  some  cells  which  combine  water- 
storage  with  strengthening  (figure  50,  /);  and  it  takes  various 
other  forms  too  many  to  mention. 

Furthermore,  the  composition  of  the  wall  is  alterable  both 
physically  and  chemically.  Cellulose  is  a  very  elastic  substance, 
and  where  greater  stiffness  than  it  can  afford  must  be  had,  the 
wall  becomes  penetrated  by  the  far  stiff er  substance  lignin;  and 
lignified  walls  are  wood.  Both  cellulose  and  lignin,  however, 
allow  ready  passage  of  water,  and  where  that  would  be  a  danger, 
as  at  surfaces  of  plants  which  grow  in  dry  air,  the  wall  is  made 
waterproof  by  the  formation  all  through  its  texture  of  a  water- 
repelling  substance,  called  cutin  or  suberin;  and  such  is  the  case 
with  the  epidermis  and  cork  which  form  the  skin  of  plants.  In 
other  cases  the  wall  softens  to  mucilage  on  the  access  of  water, 
as  hi  Flax  seeds,  though  the  reason  thereof  is  not  perfectly  clear; 
and  there  are  yet  other  such  modifications  of  more  special  char- 
acter and  meaning. 

It  is  thus  plain  that  cell-walls  are  well-nigh  indefinitely  plastic 
in  shape,  thickening,  and  composition,  while,  moreover,  any  and 
all  of  these  features  can  be  combined  in  various  ways  and  de- 
grees in  accordance  with  the  particular  needs  or  functions  con- 
cerned. Furthermore,  the  cells  are  rarely  isolated;  but  commonly 
cooperate  in  large  masses  of  similar  function  called  tissues. 
Masses  of  tissues  cooperating  in  function,  and  mutually  adjusted 
to  perform  their  work  to  the  common  advantage,  form  organs, 
and  organs  make  up  the  plant. 

There  remains  one  other  matter  of  importance  about  the  wall. 
Although,  at  first  sight,  it  seems  to  shut  off  completely  the  proto- 
plasm of  each  cell  from  that  of  its  neighbors,  minute  observation 
exhibits  the  presence  of  definite  thin  places  perforated  by  very 
fine  pores  which  permit  the  passage  of  tiny  threads  of  living  proto- 
plasm from  one  cell  to  another  (figure  51).  This  continuity  of 
protoplasm  from  cell  to  cell  has  been  found  in  every  part  of  the 


158  The  Living  Plant 

plant,  where  it  has  been  sought;  and  it  seems  clear  that  every 
living  cell  is  thus  in  communication  with  its  neighbors,  and 
therefore  with  every  other  living  cell  of  the  plant.  Thus  the 
protoplasm  though  partially,  is  not  wholly,  separated  into  cellu- 
^ ,  f;.  lar  masses,  and  is,  after  all,  for  any 

,         .         •  /     individual  plant  a  single  great  con- 

.      ,:;:;  ;  ;^     ^^i/  ,#'    tinuous  sheet.    There  is  every  reason 

/ . :\  v? ^fif?      to  believe  that  impulses  of  different 

••'•'"";     "" ";:"•'•       v/'^fW'1,     kinds  can  be  transmitted  from  cell 
';••':'•'  </nZ       to  cell  through  these  threads,  which, 

therefore,  take  the  place  in  part  of 
the  nerve  system  of  animals.     This 
-     helps  us  to  understand  how  it  is  that 
:'\r$ I   ;-^  .    :  ,t,  ;><      the  plant  can  act  as  a  physiological 

'^~^::' f'-^^^-^^^^lr^       unit:  how  the  different  parts  of  a 
plant   can  be  kept   in  harmonious 
FIG.  5i.— An  ordinary  ceil  specially  cooperation :  and  how  stimuli  applied 

treated  to   show  the  thin  threads 

of  protoplasm  extending  through  at  one  part  of  a  plant,  can  produce 
!Jith^hat^fltsln3gh£re.r0(CoS  their  effects  at  a  considerable  dis- 

from  Strasburger's  Lehrbuch). 


Thus  much  for  the  wall  of  the  cell,  to  which,  it  may  seem  to  the 
reader,  I  have  devoted  a  disproportionate  space  and  attention. 
Yet  while  vastly  less  important  than  the  protoplasm,  the  solidity, 
prominence,  and  relative  permanence  of  the  walls  makes  them  far 
more  accessible  to  study, — to  such  a  degree  indeed  that  our  con- 
ceptions of  cellular  structure  center  much  more  largely  around 
the  walls  than  the  protoplasm.*  This,  however,  is  less  unfortu- 

*  The  inconspicuousness  of  the  living  protoplasm  of  plants  in  comparison  with 
the  prominence  of  the  walls  it  builds  finds  striking  exemplification  in  the  history  of 
their  discovery ;  for  the  mass  of  the  walls  was  well  described,  and  their  cavities  were 
named  cells,  by  Robert  Hooke  as  early  as  1667,  while  they  were  elaborately  described 
and  beautifully  pictured  only  a  few  years  later,  1672-1682,  in  the  fine  books  of  Grew 
and  Malpighi.  But  the  protoplasm  was  not  recognized  at  all  as  a  constituent  of 
cells  until  over  a  century  and  a  half  later,  and  was  only  first  adequately  described 
and  named  by  von  Mohl,  in  1844. 

Here  is  Hooke's  sentence,  of  1667,  in  which  cells  were  first  named.    He  is  describ- 


The  Substance  Which  Is  Alive  in  Plants  159 

nate  than  it  might  seem,  because  the  constitution  of  the  walls 
is  so  closely  interlocked  with  the  functions  of  the  cells  that  from 
the  one  we  can  infer  much  as  to  the  other. 

Passing  now  to  the  cytoplasm  we  can  briefly  dismiss  it,  for, 
being  the  typical  protoplasm,  it  has  already  been  fully  described 
in  the  earlier  part  of  this  chapter.  It  is  the  working  body  of  the 
cell,  concerned  with  its  nutrition,  construction,  etc.,  and  the 
streaming  movements  are  probably  concerned  with  the  trans- 
portation of  substances  through  the  cell,  a  view  sustained  by  the 
fact  that  the  streaming  is  most  active  in  general  in  the  cells  which 
are  largest.  The  cytoplasm  does  not  differ  particularly  in  appear- 
ance in  different  cells,  excepting  that  it  is  more  fluid  in  some  and 
more  solid  in  others.  One  point  of  present  interest  about  it,  how- 
ever, is  this,  that  just  at  this  time  of  writing,  certain  newly  found 
tiny  bodies  within  it,  called  mitochondria,  or  chondriosomes 
are  attracting  much  attention,  and  may  prove  to  be  very  im- 
portant. 

We  come  next  to  the  nucleus  of  the  cell.    It  consists  of  living 

ing  the  appearance  presented  by  a  thin  section  of  cork  placed  under  his  microscope. 
"I  could  exceeding  plainly  perceive  it  to  be  all  perforated  and  porous,  much  like  a 
Honey-comb,  but  that  the  pores  of  it  were  not  regular;  yet  it  was  not  unlike  a  Honey- 
comb in  these  particulars. 

First,  in  that  it  had  a  very  little  solid  substance,  in  comparison  of  the  empty  cavity 
that  was  contain'd  between,  .  .  .  for  the  Inlerstitia,  or  walls  (as  I  may  so  call  them) 
or  partitions  of  those  pores  were  neer  as  thin  in  proportion  to  their  pores,  as  those 
thin  films  of  Wax  in  a  Honey-comb  (which  enclose  and  constitute  the  sexangular  cells) 
are  to  theirs. 

Next,  in  that  these  pores,  or  cells,  were  not  very  deep,  but  consisted  of  a  great  many 
little  Boxes,  separated  out  of  one  continued  long  pore,  by  certain  Diaphragms,  .  .  . 

I  no  sooner  discern'd  these  (which  were  indeed  the  first  microscopical  pores  I  ever 
saw,  and  perhaps,  that  were  ever  seen,  for  I  had  not  met  with  any  Writer  or  Person, 
that  had  made  any  mention  of  them  before  this)."  .  .  .  (Robert  Hooke,  Micro- 
Craphia,  1665,  113.) 

Here  is  von  Mohl's  sentence,  of  1844,  in  which  protoplasm  was  first  named: — "So 
mag  es  wohl  gerechtfertigt  sein,  wenn  ich  zur  Bezeichnung  dieser  Substanz  eine 
auf  diese  physiologische  Function  sich  beziehende  Benennung  in  dem  Worte  Proto- 
plasma  vorschlage."  (Botanische  Zeitung,  1844,  page  273);  or,  in  translation,  "Ac- 
cordingly it  may  be  justifiable  if  for  designating  this  substance  I  propose  an  appellation 
having  reference  to  this  physiological  function,  namely,  the  word  Protoplasm." 


160  The  Living  Plant 

protoplasm,  denser  than  the  cytoplasm,  and  different,  somewhat, 
chemically.  It  varies  comparatively  little  in  appearance  in  differ- 
ent cells,  and  ordinarily  exhibits  no  particular  structure;  but  when 
the  cells  are  dividing  or  reproducing,  then  a  definite  number  of 
rod-shaped  structures  become  differentiated  and  perform  re- 
markable manoeuvres  which  we  shall  later  consider  in  the  suit- 
able place  along  with  reproduction  (figure  101) .  These  rods,  called 
chromosomes,  are  the  seat  of  the  controlling  power  of  heredity, 
and  thus  guide  the  constructive  work  of  the  cytoplasm  in  growth. 
The  nucleus,  therefore,  bears  to  the  cytoplasm  a  relation  sug- 
gestive of  that  of  the  brain  to  the  body.  Indeed,  the  resemblance 
may  extend  pretty  far,  since  there  are  those  who  maintain  that 
heredity  in  the  chromosomes  is  substantially  the  same  thing  as 
memory  in  the  brain.  But  I  hope  the  reader  will  not  therefore 
call  the  nucleus  the  brain  of  the  cell,  for  it  isn't.  As  to  the  nu- 
cleolus,  that  is  irregular  hi  its  appearance,  and  probably  repre- 
sents a  reserve  of  chemical  substance  for  use  in  the  growth  of  the 
chromosomes. 

The  plastids,  likewise,  are  living  protoplasm,  and  are  present  in 
all  cells  of  the  typical  plants,  though  sometimes  they  are  incon- 
spicuous. Thus,  it  is  the  plastids  which  hold  the  green  color 
in  leaf-cells,  where  they  are  already  well  known  to  the  reader  as 
chlorophyll  grains,  called  also  chloroplastids.  In  other  cells, 
of  some  fruits,  such  as  the  familiar  Jerusalem  Cherry,  they  con- 
tain yellow  or  orange  colors  (chromoplastids),  thus  aiding  to 
make  the  fruits  conspicuous.  And  in  other  cells  yet,  especially 
in  the  storage  parts  of  plants,  they  remain  colorless  and  are  called 
leucoplastids,  but  perform  the  remarkable  and  indispensable 
function  of  converting  sugar  to  starch.  It  is,  indeed,  a  fact  of 
the  greatest  interest  about  these  leucoplastids,  that  they  and  the 
homologous  chloroplastids  comprise  the  only  places  in  nature, 
either  within  the  plant  or  outside  of  it,  where  starch  is  known  to  be 
made.  Starch  is  one  of  the  substances  which  the  chemist  has  not 
yet  been  able  to  make  in  his  laboratory. 


The  Substance  Which  Is  Alive  in  Plants  161 

The  sap-cavities  are  as  simple  in  structure  as  they  look.  In 
very  young  cells  they  are  absent,  as  a  later  picture  illustrates 
(figure  129) ;  but  in  those  that  are  older  little  rifts  appear  in  the 
cytoplasm  and  gradually  grow  larger  until  finally,  in  the  fully 
mature  cell,  they  become  merged  into  a  single  cavity  of  very  large 
size,  as  the  figure  of  the  conventionalized  cell  clearly  shows 
(figure  48).  This  cavity  is  filled  with  water  in  which  sugar  and 
other  useful  substances  are  dissolved.  It  thus  represents  a  store- 
house of  useful  materials,  but  serves  secondary  functions,  like- 
wise, in  pressing  the  cytoplasm  against  the  wall,  and  in  aiding 
growth,  by  methods  which  will  later  be  described  in  the  suitable 
chapters. 

It  is  of  interest  to  note  that  not  only  does  new  protoplasm  in 
general  originate  only  from  preexisting  protoplasm,  but  new 
cells  originate  only  from  cells,  nuclei  from  nuclei,  and  plastids 
from  plastids;  while  the  same  thing  has  been  claimed  even 
for  cell-wall  and  sap-cavities,  or  rather  for  the  part  of  the 
cytoplasm  which  forms  them,  though  here  the  evidence  is  not 
conclusive. 

The  question  is  now  appropriate,  why  does  protoplasm  sep- 
arate into  cells  at  all,  and  what  makes  them  of  such  minute  size 
as  they  are?  It  is  sometimes  assumed  that  the  plant-structure 
becomes  cut  up  into  cells  in  order  to  provide  structural  units  of 
convenient  size  and  form,  after  the  manner  of  the  bricks  of  the 
builder;  but  the  analogy  is  wholly  misleading,  since  the  skeleton 
of  plants  is  not  built  at  all  from  originally  separate  units,  as  brick 
buildings  are,  but  rather  from  a  continuous  mass  of  cell-wall 
substance  comparable  with  the  cement  construction  now  coming 
into  use.  Another  explanation  maintains  that  each  nucleus 
can  control  only  a  limited  quantity  of  cytoplasm;  and  thus  are 
established  certain  administrative  units  between  which,  natu- 
rally enough,  the  walls  are  built, — the  resultant  being  cells.  As 
to  the  reasons  why  their  sizes  are  so  small  as  to  require  a  mi- 
croscope to  show  them  at  all,  we  have  again  a  few  guesses,  but  no 


1 62  The  Living  Plant 

exact  knowledge.  Possibly  the  chromosomes  need  a  certain 
size  in  order  to  perform  their  functions;  this  would  establish  the 
size  of  the  nucleus,  and  hence  (on  the  explanation  above  noted) 
of  the  cell.  Another  explanation  rests  on  a  mathematical  basis. 
We  may  assume  that  the  typical  cell  is  a  sphere  filled  solidly 
with  protoplasm.  When  a  sphere  enlarges  in  size,  its  bulk  in- 
creases much  faster  than  its  surface,  the  bulk  increasing  as  the 
cube  of  its  diameter  and  the  surface  as  the  square  thereof.  Ob- 
viously it  is  through  this  surface  that  the  spherical  cell  must 
absorb  the  oxygen  for  the  respiration  of  the  entire  bulk  of  its 
protoplasm;  it  is  therefore  quite  evident  that  there  must  be  a 
certain  size  of  the  cell  in  which  the  surface  is  just  sufficient  to 
aerate  the  bulk  of  protoplasm  within, — and  that  size  would  de- 
termine the  average  cell  size.  If  the  cell  were  to  grow  larger  its 
surface  would  not  suffice  to  aerate  the  bulk,  while  if  smaller  the 
surface  would  be  needlessly  great.  In  a  general  way  this  con- 
clusion is  sustained  by  the  fact  that  where  conditions  for  respira- 
tion are  harder  the  cells  are  smaller,  and  vice  versa.  Moreover, 
the  very  largest  cells  occur  in  places  well  situated  for  aeration, 
and  besides,  possess  accessory  arrangements, — viz.,  the  flattening 
of  the  protoplasm  in  a  thin  layer  against  the  wall  (figure  45) ,  and 
protoplasmic  streaming, — which  aid  to  that  end.  These  features 
prevail  in  the  hair  cells  already  observed  by  the  reader,  and  in 
consequence  those  cells  become  large  enough  to  be  visible  to  the 
eye  without  the  aid  of  a  lens.  In  general,  therefore,  it  does  seem 
true  that  the  relation  of  bulk  to  surface  in  a  solid  as  affecting  res- 
piration is  one  of  the  principal  factors,  if  not  indeed  the  principal 
one,  in  making  the  size  of  cells  what  it  is. 

In  comparing  the  functions  of  the  cells  of  plants  with  those  in 
animals,  it  soon  becomes  obvious  that  plant  cells  exhibit  a  far 
lower  degree  of  division  of  labor;  and  this  involves  a  remarkable 
consequence.  It  seems  to  be  a  fact  that  when  protoplasm  con- 
tinues to  perform  a  single  function  for  long  periods  of  time,  as  it 
does  in  the  highly-specialized  organs  of  the  animal  body,  it  grows 


The  Substance  Which  Is  Alive  in  Plants  163 

stronger  and  stronger  and  works  better  and  better  up  to  a  certain 
culminating  level,  beyond  which  it  tends  to  decline,  and  finally 
to  cease  work  altogether.  It  is  probable  that  the  decline  and 
cessation  of  work  is  dependent  upon  purely  physical  causes,  some- 
what as  a  bar  of  metal  when  too  often  bent,  becomes  weakened 
and  broken  at  last;  but  in  this  peculiarity  of  protoplasm  we  find 
an  explanation  for  the  cycle  of  youth,  maturity,  old  age  and  death. 
When,  however,  the  protoplasm  can  periodically  alter  its  location, 
habits,  or  functions, — can  re-melt  itself,  so  to  speak, — it  renews 
its  youth  thereby,  and  can  continue  its  vigor  without  limit,  thus 
becoming  potentially  immortal.  In  this  fact  is  found  the  ex- 
planation of  the  benefit  wrought  by  a  change  of  scene  or  occupa- 
tion, or  a  vacation,  upon  ourselves,  though  the  effect  is  here 
limited;  and  if  a  way  could  be  found  to  affect  our  protoplasm 
more  profoundly, — to  make  it  mix  itself  up  periodically,  even 
within  the  limits  of  the  same  cell, — then,  it  seems  likely,  man 
would  have  discovered  the  long-sought  elixir  of  life  and  the 
secret  of  perpetual  youth.  This  in  fact  is  the  case  in  full  degree 
in  simple  plants  like  Bacteria.  Each  of  these  is  made  of  one  cell, 
and  when  it  reaches  full  size  divides  into  two,  each  of  which 
grows  up  and  divides  again,  and  so  on  without  limit,  in  perennial 
change,  vigor,  and  youth.  A  similar  rejuvenation  takes  place  in 
sexual  reproduction,  when  the  protoplasm  of  two  individuals 
mingles  together  in  fertilization.  Now,  the  higher  plants  possess 
no  organs  at  all  in  which  the  protoplasm  continues  to  work  within 
the  same  cells  throughout  the  life  of  the  individual,  but,  as  our 
chapter  on  growth  will  abundantly  illustrate,  the  protoplasm  is 
continually  moving  outward  and  onward  into  newly  forming 
buds,  leaves,  roots,  and  stems;  and  this  removal  permits  it  to  re- 
new its  youth  perennially.  Therefore  plants  should  never  grow 
old  from  internal  causes,  in  the  way  that  animals  do, — and  in 
fact  they  do  not,  the  exception  presented  by  annuals  being  only 
apparent  and  not  real.  Even  the  greatest  trees  continue  to  form 
new  leaves  and  roots  with  unabated  vigor  until  they  are  brought 


164  The  Living  Plant 

to  their  death  by  external  causes,  chiefly  connected  with  the 
large  size  they  attain. 

The  manifestations  of  life,  wherever  we  know  them,  are  as- 
sociated closely  with  constant  changes  of  matter  and  energy, 
especially  with  respiration.  But  there  is  a  case  in  which  all  of 
these  processes  seem  suspended,  for  our  most  delicate  methods  of 
research  fail  to  demonstrate  them,  and  that  is  in  resting  struc- 
tures such  as  seeds,  Resurrection  plants,  and  some  low  animals. 
Not  only  can  dry  seeds  retain  their  vitality  for  a  great  many  years, 
but  in  that  condition  they  can  withstand  without  injury  a  tem- 
perature above  boiling  point,  or  even  two  hundred  degrees  below 
freezing  point.  The  question  is  important  whether  the  usual 
changes  are  proceeding  in  these  seeds,  but  too  slowly  to  be  meas- 
ured, or  whether  all  processes  stop  and  the  vitality  is  really  sus- 
pended. The  truth  is  not  as  yet  known,  but  it  is  to  be  noted  that 
there  is  no  logical  difficulty  in  supposing  that  all  of  the  processes 
may  slow  down  to  a  stop  without  any  derangement  of  machinery, 
precisely  as  an  engine  is  stopped  for  the  night  simply  by  with- 
holding the  steam,  leaving  it  all  ready  to  start  once  more  in  the 
morning. 

When  this  chapter  was  finished  down  to  this  point,  it  was 
handed  like  all  of  the  others  to  a  critic  for  judgment.  And  this 
is  in  substance  the  comment  with  which  it  came  back.  "The 
chapter  is  clear  enough  in  its  statements,  and  appears  to  cover 
the  subject,  but  somehow  it  leaves  you  with  a  very  unsatisfied 
feeling."  This  opinion  I  take  for  a  very  high  compliment,  since 
it  shows  that  my  chapter  reflects  precisely  the  scientific  situation 
of  the  subject. 


CHAPTER  VII 

THE  WAYS  IN  WHICH  PLANTS  DRAW  INTO  THEMSELVES 
THE  VARIOUS  MATERIALS  THEY  NEED 

Absorption;  Roots 


N  the  preceding  chapters  we  have  traced  pretty  fully 
the  principal  processes  occurring  within  the  bodies  of 
plants.  But  as  yet  we  have  taken  no  thought  of  the 
ways  in  which  plants  absorb  the  various  materials  they 
need  from  outside;  and  this  is  the  inquiry  which  now  lies  before 
us.  I  give  the  reader  fair  warning  that  the  subject  will  lead  us 
perforce  into  distant,  unfamiliar,  and  recondite  matters;  but  their 
study  will  have  the  advantage  of  illuminating  a  good  many  things 
besides  absorption  by  plants. 

Of  all  the  substances  absorbed  by  plants,  the  foremost  is  water, 
which  not  only  makes  up  a  great  part  of  plant  substance,  but 
is  also  indispensable  for  various  physical  and  chemical  uses. 
This  water  is  absorbed,  as  everybody  knows,  through  the  roots; 
but  the  fact  is  less  familiar  that  the  absorption  takes  place  ex- 
clusively through  the  young  white  terminal  parts.  Upon  these, 
accordingly,  we  now  center  our  attention.  They  can  be  studied 
most  easily,  and  without  obscuration  by  adherent  soil  particles, 
in  young  roots  obtained  by  the  germination  of  seeds  in  flower-pot 
saucers  kept  shaded  and  wet.  From  such  specimens  it  appears 
that  young  roots  as  a  whole  are  remarkably  alike,  and  possess 
several  features  in  common, — viz.,  a  slender  white  shaft  with  a 
yellowish  tip,  and  a  diaphanous  garment  of  delicate  radiating 
hairs, — features  which  are  shown  very  well,  except  for  the  color, 
in  the  seedling  of  Mustard  pictured  herewith  (figure  52).  If 

165 


1 66  The  Living  Plant 

one  centers  observation  more  exactly  on  these  hairs,  he  will  see 
that  nearest  the  tip  they  are  plainly  just  forming,  while  farther 
back  they  are  progressively  longer,  until  a  maximum  is  reached, 
behind  which  they  are  obviously  withering  and  dying.  Evi- 
dently the  single  hairs  have  each  their  little  day 
and  pass,  while  the  zone  as  a  whole  moves  forward 
in  perpetual  youth,  pari  passu  with  the  advancing 
tip  of  the  root.  These  hairs  are  of  first  importance 
to  our  immediate  subject,  for  they  are  the  active 
water-absorbing  parts  of  the  roots. 

Thus  much  can  be  seen  with  the  eye  and  a 
lens,  but  hardly  anything  more.  If,  however,  one 
cuts  a  thin  section  through  the  apex  and  along  the 
central  axis  of  a  root,  and  magnifies  this  section 
with  the  microscope,  he  will  have  before  him  an 

FIG.  52.— A  seed- 
ling of  mustard  arrangement  like  that  of  our  picture  (figure  53) , 

grown  in   dark         .,,  ,  .    ,      .          .,,     ,  ,      .      ,  ,  ,«• 

saturated  air;  with  which  it  will  be  ^desirable  to  compare  the 
natural  size.  generalized  section  of  figure  139  C.  At  the  tip  is 
the  root-cap,  a  cluster  of  cells  which,  continually  renewed  from 
behind,  acts  as  a  protection  to  the  delicate  tip  in  its  passage 
through  the  rough  and  abrading  soil;  just  behind  lies  a  prominent 
focal  center,  the  growing  point,  whose  closely-packed  cells  are  so 
densely  filled  with  protoplasm  that  the  characteristic  yellowish 
color  of  that  substance  shows  through  to  the  outside;  while  radi- 
ating back  from  the  growing  point  run  long  lines  of  cells  which 
gradually  merge  into  the  differentiated  tissues  of  the  older  root. 
These  latter  tissues,  so  far  as  they  concern  our  immediate  subject 
of  absorption,  are  shown  generalized  in  figure  54,  D.  Of  the  lines 
of  cells,  a  few  in  the  center  constitute  the  pith,  outside  of  which  lie 
the  long  lines  of  water-tubes,  or  ducts,  readily  identified  by  their 
distinctive  spiral  markings.  These  ducts  contain  water,  but, 
contrary  to  what  one  would  expect,  are  otherwise  empty  tubes, 
possessing  no  living  protoplasm  after  once  they  are  formed;  and 
they  run  in  continuous  strands  from  the  tips  of  the  roots  all 


FIG.  53.— Typical  parts  of  a  section  drawn  lengthwise,  cell  for  cell,  through  a  young 
root   of   Corn.     The   entire  section   is   not   presented   because   its   length  would  be 
much  too  great  for  the  page.     The  tip  portion  is  magnified  more  than  the  others. 
167 


D.    Longitudinal  section  through  a  portion  of  root  at  *. 


Fio.  54. — Generalized  drawings  illustrating  the  absorbing  and  conducting  systems  of  the 

plant. 


168 


How  Plants  Draw  in  Various  Materials 


169 


through  the  stems  to  the  leaves,  as  shown  very  clearly  in  the  con- 
ventionalized plant  of  figure  54,  A.  Outside  of  the  ducts  lie  some 
rows  of  rounded-elongated  cortical 
cells,  each  of  which  retains  its  lining 
of  protoplasm  and  is  shown  by  tests 
to  contain  a  solution  of  sugar.  Finally, 
outside  of  all  lies  the  single  thin  line 
of  epidermal  cells,  which  display  a 
very  striking  feature,  viz.,  a  great 
many  are  prolonged  into  slender  cy- 
lindrical closed  tubes,  which  are  ob- 
viously identical  with  the  root  hairs  al- 
ready observed  in  the  young  living 
roots;  and  each  hair  is  lined  by  living 
protoplasm  and,  as  shown  by  suitable 
tests,  contains  a  solution  of  sugar. 
Now  the  structures  important  from 
the  view  of  water-absorption  are  the 
ducts,  the  cortical  cells,  and  the  root 
hairs;  and  these  parts  constitute  the 
water-absorbing  machine.  And  this 
machine,  if  reduced  to  a  single  cell  of 
each  kind,  would  be  constructed  some- 
what as  suggested  by  figure  55. 

We  turn   now   to  the  forces    con- 
cerned in  absorption.     Most  people, 

if  questioned,  WOUld  doubtless  express     FlG-  55.— A  diagram  of  the  con- 
struction of  the  water-absorbing 

the  belief  that  roots  suck  up  water  in 
much  the  same  manner  that  a  wick 
sucks  up  oil,  that  is,  by  the  power, 
called  in  physics  capillarity, — the  same 
which  takes  liquids  up  fine  tubes.  In- 
deed this  was  once  the  belief  of  botanists  themselves,  as  witnessed 
by  their  former  use  of  the  term  "spongiole, "  that  is  "  little 


machine,  as  it  would  appear  if 
reduced  to  a  single  root  hair, 
cortical  cell,  and  duct.  Pro- 
toplasm is  shaded;  circles  are 
water;  crosses  are  sugar;  the  ar- 
rows show  direction  of  move- 
ment. Magnification  as  in  Fig.  6. 


i  yo  The  Living  Plant 

sponge,"  for  the  tip  of  the  root,  which  was  supposed  to  soak  up 
water  and  pass  it  on  to  the  ducts.  Later  it  was  found  that  the 
water  enters  chiefly  through  the  hairs.  But  in  these  the  condi- 
tions for  capillarity  are  absent;  for  capillarity  requires  openings, 
and  the  hair  walls  contain  none  that  even  the  most  powerful  mi- 
croscopes can  detect.  The  problem  is,  therefore,  to  explain  an 
absorption  of  water  through  membranes  that  are  imperforate,  or 
solid,  and  through  protoplasm-lined  and  sugar-holding  cells  into 
protoplasm-less  and  sugar-less  ducts.  But  the  very  mention  of 
absorption  into  sugar  solutions  through  imperforate  membranes 
immediately  suggests  a  direction  for  our  further  inquiry,  since  it 
recalls  a  mode  of  absorption  very  well  known  in  physics,  and  as- 
sociated with  those  very  conditions,  viz.,  Osmosis. 

So  important  is  this  subject  of  osmosis  to  an  understanding  not 
only  of  absorption  of  water  by  plants,  but  of  many  other  notable 
phenomena  as  well,  that  the  reader  ought  really  to  make  its  more 
intimate  personal  acquaintance.  This  he  can  do  by  aid  of  the 
following  experiment,  which  is  one  familiar  to  all  workers  with 
plant  physiology.  Over  the  end  of  a  large  glass  tube  is  tied 
firmly,  by  means  of  waxed  thread,  a  piece  of  soaked  parchment, 
(preferably  a  cylindrical  parchment  cup  made  for  the  purpose) 
which  is  a  physical  equivalent  of  the  wall  of  the  root  hair;  into  the 
tube  is  poured  a  solution  of  sugar,  for  which  molasses,  a  solution 
ready  made  and  conveniently  colored,  is  excellent ;  then  the  tube 
is  supported  with  the  parchment  in  pure  water,  the  whole  arrange- 
ment being  much  as  displayed  in  the  accompanying  picture 
(figure  56).  A  surprising  result  always  follows,  for,  without  the 
operation  of  any  visible  machinery  or  forces,  and  in  a  manner 
which  to  me,  despite  long  familiarity  therewith,  looks  always 
anomalous  and  even  somewhat  uncanny,  the  liquid  rises  steadily 
though  slowly  in  the  tube,  lifting  its  own  considerable  weight, 
until  within  two  or  three  days  it  has  reached  a  height  of  two  or 
three  feet  or  more.  Indeed  the  process  can  be  demonstrated 
even  more  strikingly  than  this,  for,  if  the  parchment  cup  be 


How  Plants  Draw  in  Various  Materials 


171 


made  very  large  and  the  glass  tube  very  small,  as  is  readily  ar- 
ranged for  purposes  of  demonstration,  the  liquid  will  mount  stead- 
ily up  before  the  very  eyes  to  a  height  of  several  feet.  Obviously 
there  is  only  one  possible  explanation  of  the  rise  of  the  liquid 
against  gravitation, — viz.,  water  must  pass  through  the  parch- 


FIQ.  56. — An  osmoscope,  using  a  parchment  membrane;  further  particulars  in  text. 

ment,  and  that  not  simply  in  a  manner  that  is  passive,  but  with 
a  force  sufficient  to  overcome  a  considerable  resistance.  The  same 
result  invariably  follows,  with  a  difference,  however,  in  the  rate 
of  the  ascent,  no  matter  what  solution  is  put  inside  of  the  tube, 
and  follows,  moreover,  in  case  there  is  also  a  solution  outside,  if 
only  the  inner  solution  is  the  stronger.  This  is  a  typical  example 
of  osmosis  under  its  simplest  conditions,  but  it  is  representative  of 


172 


The  Living  Plant 


all  conditions,  inside  of  plants  and  animals,  as  well  as  outside  of 
them.  It  thus  constitutes  one  of  the  great  natural  verities  which 
may  be  stated  as  follows; — when  water  and  a  solution,  or  two  solu- 
tions of  different  strengths,  are  separated  by  a  suitable  membrane, 

there  is  always  a  forcible  osmotic 
movement  of  liquid  through  the  mem- 
brane from  the  weaker  to  the  stronger 
solution.  This  is  one  of  those  ele- 
mental cosmical  facts  which  the 
reader  should  fix  in  his  mind  as  one 
of  the  pillars  of  his  natural  knowl- 
edge. 

If,  at  this  point,  it  seems  to  the 
reader  that  however  interesting  such 
experiments  with  parchment  and 
tubes  may  be,  they  can  have  little 
to  do  with  the  processes  inside  of  a 
living  plant,  let  him  take  a  leafy 
potted  Begonia,  Fuchsia,  or  Mar- 
guerite, cut  everything  away  close 
down  to  the  roots,  and  connect  the 
stump  with  a  plain  glass  tube  like 
that  which  was  used  in  the  foregoing 
experiment.  Then,  I  believe,  he  will 
change  his  opinion,  for  water  always 
rises  in  the  tube,  though  slowly,  to 
a  height  of  two  or  three  feet  (figure 
57).  There  are  of  course  plenty  of 

differences  in  detail,  but  sugar-holding  cup  and  live  roots  agree  in 
the  central  and  crucial  feature  that  they  absorb  water  into  a  sugar 
solution  through  imperforate  membranes  and  force  it  up  tubes 
against  gravitation.  There  is  no  question  that  the  primary 
forces  are  the  same  in  both  cases,  and  that  the  absorption  of 
water  by  roots  is  osmotic. 


How  Plants  Draw  in  Various  Materials  173 

We  return  for  a  moment  to  our  osmoscope,  for  such  is  the  name 
of  our  osmosis-exhibiting  instrument.  As  the  liquid  ascends  in 
the  tube,  a  brown  color  appears  in  the  water  outside,  showing 
that  some  of  the  molasses  comes  out,  though  of  course  in  much 
smaller  amount  than  the  water  which  enters,  else  the  liquid  could 
not  rise  in  the  tube.  This  suggests  at  once  the  inquiry, — does  the 
sugar  in  the  sap  of  the  root-hair  cells  also  come  out  into  the  soil? 
It  does  not,  as  ample  evidence  attests.  And  if  we  seek  in  parch- 
ment cup  and  root  hair  for  a  structural  difference  to  explain  this 
difference  in  osmotic  action,  we  can  easily  find  it;  for  the  hairs 
possess  a  complete  lining  film  of  living  protoplasm  to  which 
there  is  no  equivalent  in  the  parchment  cup.  It  is  easily  shown  by 
experiment  that  this  protoplasm  really  does  stop  the  passage  of 
sugar  while  permitting  that  of  water;  and  this  fact  explains  not 
only  why  no  sugar  passes  out  of  the  hairs  into  the  soil,  but  also 
the  equally  striking  phenomenon,  that  none  passes  out  of  the 
cortical  cells  into  the  ducts,  for  in  general  it  is  only  pure  water 
which  ascends  through  the  ducts  to  the  leaves.  Protoplasm, 
however,  is  not  the  only  membrane  of  this  type  (which,  because 
permeable  to  water  but  not  to  dissolved  substance  is  called  semi- 
permeable,  in  distinction  from  the  ordinary  kind  which  are  per- 
meable to  both),  for  they  can  be  constructed  artificially  from  chem- 
icals, and  even  laid  down  in  a  uniform  film  all  over  the  interior 
face  of  the  parchment  cup.  In  this  case  our  osmoscope  becomes  a 
very  close  physical  duplicate  of  the  living  root  hair,  and  likewise 
permits  the  steady  absorption  of  water  without  the  escape  of  any 
of  the  sugar  whatsoever.  My  students  have  often  constructed 
such  arrangements,  with  results  that  were  wholly  satisfactory. 

If,  now,  the  reader  will  compare  point  by  point,  an  osmoscope 
containing  a  semipermeable  membrane,  and  the  absorbing  mech- 
anism of  the  living  root  (which  is  diagrammatically  represented 
in  figure  55),  he  will  agree  that  they  match  very  closely  in  physical 
construction  and  operation  except  for  one  very  notable  difference, 
— namely,  while  the  liquid  which  rises  in  the  osmoscope  is  a  mix- 


i/4  The  Living  Plant 

ture  of  molasses  and  water,  that  which  rises  in  the  ducts  is  prac- 
tically pure  water.  This  difference,  obviously,  is  correlated  with 
a  difference  of  structure,  viz.,  in  the  plant  the  water  has  to  pass 
through  intermediate  cells,  which  are  wanting  in  the  osmo- 
scope.  We  have  already  learned  why  it  is  that  the  sugar  does 
not  pass  with  the  water  into  the  ducts  (the  protoplasm  stops 
it),  and  our  problem  resolves  itself  into  this, — how  is  it  that 
the  cortical  cells  send  water  into  the  ducts  when  all  of  the  con- 
ditions seem  rather  to  invite  the  absorption  of  water  from  them, 
exactly  as  the  hairs  absorb  it  from  the  soil?  This  question, 
I  am  sorry  to  say,  I  cannot  yet  answer,  for  it  remains  one  of  the 
unsolved  problems  of  plant  physiology,  though  one  of  the  most 
inviting  of  them  all.  It  is  true,  some  physiological  books  at- 
tempt to  explain  it,  but  in  all  cases,  so  far  as  I  have  observed, 
either  their  physics  is  bad,  or  else  their  explanations  are  worded 
in  a  manner  more  lethal  than  logical.  I  suspect  the  explana- 
tion will  ultimately  be  found  in  some  ordinary  physical  or  chem- 
ical processes  working  under  control  of  some  still  unknown  prop- 
erty of  the  protoplasm. 

In  the  three  or  four  paragraphs  which  follow  I  purpose  to 
explain  how  it  is  that  substances  in  solution  can  pass  through 
imperforate  membranes,  and  what  are  the  forces  which  drive 
them.  The  subject  involves  a  consideration  of  molecules,  and 
things  of  that  sort,  and  will  require  hard  work  from  the  con- 
structive imagination.  So  the  reader  is  given  fair  warning  and 
may  skip  if  he  pleases,  though  I  beg  to  remind  him  that  this 
book  makes  appeal  to  his  reason,  and  is  an  attempt  to  help  him 
to  share  the  spartan  pleasures  of  understanding. 

The  most  striking  feature  of  osmotic  absorption  consists  in 
the  remarkable  rise  of  a  large  body  of  liquid  against  gravitation 
without  the  operation  of  any  visible  forces  whatsoever.  Yet 
forces  there  must  be,  if  not  visible,  then  invisible;  and  accord- 
ingly we  turn  for  the  sources  of  the  power  deep  within  the  con- 
stitution of  the  bodies  themselves.  As  everybody  knows,  mem- 


How  Plants  Draw  in  Various  Materials  175 

branes,  water,  and  dissolved  substances  are  all  of  them  composed, 
according  to  the  teaching  of  physics,  of  ultimate  excessively 
small  units,  called  molecules.  In  the  solid  or  liquid  state,  the 
molecules  are  held  together  by  a  force  of  mutual  attraction, 
called  cohesion,  analogous  to  the  force  which  holds  an  armature 
to  a  magnet.  But  when  heat  in  sufficient  amount  is  supplied, 
there  comes  a  point  at  which  the  cohesion  of  the  molecules  is 
suddenly  overcome  and  replaced  by  an  opposite  tendency  to 
spread  or  diffuse  just  as  far  apart  as  they  can;  and  this  is  what 
constitutes  a  gas.  The  power  that  actuates  the  diffusion  is  heat, 
which,  catching  the  tiny  molecules  in  the  swirl  of  the  violently- 
vibratory  ethereal  waves  of  which  it  consists,  imparts  to  them 
its  own  vigorous  motion,  whereby  they  are  set  swiftly  darting 
and  dancing  hither  and  yon,  bounding  and  rebounding  energet- 
ically against  one  another,  with  a  result  that  they  work  steadily 
outward,  very  much  as  a  cargo  of  corks  would  be  spread  from  a 
foundered  vessel  on  the  waves  of  a  tempestuous  sea.  Familiar 
examples  of  this  diffusion  of  gases  are  many, — for  instance,  the 
spread  and  ultimate  disappearance  of  odors,  and  the  penetration 
of  cigar  smoke  though  the  house;  but  all  gases  diffuse  in  this 
manner.  And  here  comes  a  curious  and  consequential  fact  about 
diffusion,  namely,  that  it  occurs  not  only  in  gases,  but  also  in 
anything,  whether  solid,  liquid  or  gaseous,  when  dissolved  in  a 
liquid.  Examples  thereof  are  abundant, — the  gradual  spread  of 
a  bit  of  solid  dye  when  dropped  into  water:  the  spread  of  sugar 
through  coffee  or  tea  without  stirring  if  only  tune  be  allowed: 
the  spread  of  fertilizers  evenly  through  soil  though  added  in  large 
lumps  on  the  surface.  By  diffusion,  also,  the  molasses  reaches 
the  water  outside  of  the  tube  of  our  osmoscope.  Such  diffusion 
occurs,  as  it  seems,  because  an  adhesive  attraction  existing  be- 
tween the  molecules  of  the  substance  and  those  of  the  dissolving 
liquid  separates  the  molecules  of  the  substance  from  one  another, 
and  thus  brings  them  into  a  condition  such  that  heat  can  exert 
upon  them  the  same  action  as  it  does  upon  the  separated  mole- 


76 


The  Living  Plant 


Fio.   58. — A  diagram  designed  to  illus-    inside 
trate  the   diffusion  of  a  substance  in 
solution.     The  circles  are  water,  and    Outside 
the  crosses  are  the  dissolving  and  dif- 
fusing substance, — e.   g.,   sugar.     The 
molecules  of  water  are  supposed    to 
have    a    stronger    attraction    for    the 
molecules  of  sugar  than  these  have  for 
one  another.     Magnified  as  in  Fig.  6. 


cules  of  a  gas  (figure  58).  And  if  the  reader  objects  at  this 
point  that  diffusion  in  a  solution  takes  place  at  a  temperature 
too  low  to  permit  this  explanation,  I  remind  him  that  days  far 
too  cold  for  our  comfort  are  yet  hot  from  the  physical  point  of 

view,  for  there  is  heat  in  the  air 
at  all  temperatures  above  the  ab- 
solute zero,  which  lies  no  less  than 
four  hundred  and  fifty-nine  de- 
grees below  zero  of  our  ordinary 
thermometer.  And  the  phenomena 
of  diffusion  are  precisely  the  same 
of  plants  and  animals  as 
of  them.  We  are  now 
prepared  to  summarize  diffusion 
as  another  verity  of  nature,  thus, 
— when  substances  are  anywhere 
brought  into  a  state,  whether  by 
conversion  to  a  gas  or  by  solution  in  a  liquid,  such  that  their  mole- 
cules are  separated  from  one  another,  then  those  molecules,  set  into 
energetic  action,  and  thereby  given  a  mutually-repulsive  motion,  by 
heat  derived  from  the  surroundings,  spread,  or  diffuse,  forcibly  out- 
ward from  places  of  greater  to  those  of  lesser  concentration. 

Thus  much  for  diffusion;  we  turn  next  to  the  other  condition 
involved  in  osmosis, — the  nature  of  the  membrane.  What  can 
be  the  constitution  of  a  body  which,  possessing  no  discoverable 
openings,  will  permit  water  and  other  substances  to  pass  through 
with  a  freedom  well-nigh  as  uncanny  as  4f  a  fourth  dimension 
were  concerned?  The  membrane,  of  course,  is  composed  of  mole- 
cules, but  there  is  also  good  reason  to  believe  that,  in  walls  at  least, 
the  membrane  is  composed  of  larger  units,  called  micella,  which 
are  aggregates  of  molecules  (or  perhaps  simply  huge  compound 
molecules)  that  may  be  represented  diagrammatically  as  cubical 
(figure  59).  Now  these  micellae,  although  structurally  separate, 
are  held  closely  together  by  virtue  of  a  certain  cohesive  affinity 


How  Plants  Draw  in  Various  Materials 


177 


for  one  another,  somewhat  as  a  magnet  and  its  armature  are 
held  together  by  magnetism;  and  this  explains  why  the  mem- 
brane, although  composed  wholly  of  separate  units,  holds  to- 


FIG.  59. — A  diagram  illustrating  the  construction  of  membranes.  The  circles  are  water; 
the  smaller  squares  are  molecules,  and  the  larger  are  micella?,  of  wall  substance,  a,  rep- 
resents a  dry  membrane  (which  always  contains  some  water)  and  b,  a  saturated  mem- 
brane, supposed  to  be  seen  in  section,  reduced  to  only  a  few  micellae. 

gether  as  a  solid.  At  the  same  time  the  micellae  possess  a  still 
stronger  affinity  for  something  quite  different,  namely  water, 
which  accordingly  they  can  draw  in  as  thin  films  among  and 


178  The  Living  Plant 

around  themselves,  thus  forcing  themselves  apart  against  the 
resistance  of  their  own  cohesion.  This  explains  how  it  is  that 
membranes,  and  all  bodies  of  similar  constitution,  like  wood,  can 
forcibly  absorb  water  throughout  all  of  their  structure,  and 
swell  up  in  the  process,  the  requisite  energy  being  supplied  by 
the  adhesive  attraction  between  water  and  wood.  This  inter- 
micellar  absorption  of  water  is  called  imbibition,  and  is  represented 
in  the  accompanying  diagram  (figure  59).  But  why,  by  the  way, 
are  the  micellae  not  driven  entirely  apart  by  the  water,  thus 
making  the  membrane  completely  soluble  therein?  The  reason 
is  believed  to  be  this, — that  while  the  adhesive  attraction  of 
micellae  for  water,  and  the  cohesive  attraction  of  the  micellae  for 
one  another,  are,  like  the  attraction  of  a  magnet  for  its  armature, 
strongest  when  the  parts  are  the  closest  and  weaker  with  increas- 
ing distance  apart,  the  adhesion  is  supposed  to  weaken  with 
distance  more  rapidly  than  the  cohesion;  hence,  although  the 
adhesion  between  micellae  and  water  is  at  first  stronger  than  the 
cohesion  of  the  micellae  (thus  drawing  in  some  films  of  the  water) 
there  soon  comes  a  point  at  which  the  rapidly-lessening  adhesion 
between  water  and  micellae  exactly  balances  the  slowly-lessening 
cohesion  between  'the  micellae,  and  this  point  of  equilibrium  is 
that  where  the  membrane  is  saturated  with  water  and  swollen 
its  greatest,  as  supposed  to  be  represented  in  figure  59,  6.  In  this 
condition  the  intermicellar  spaces  will  possess  a  certain  definite 
size,  differing,  of  course,  with  the  nature  of  the  membrane;  and 
in  these  different  sizes  we  find  the  simplest  explanation  of  the 
different  behavior  of  the  types  of  membranes,  for  the  semi- 
permeable  would  be  one  with  intermicellar  spaces  too  small  to 
allow  the  sugar  or  other  large  molecules  to  pass,  while  giving  free 
transit  to  the  much  smaller  water  molecules,  while  the  permeable 
has  large  enough  spaces  to  permit  both  kinds  to  pass.  The  case 
in  reality,  however,  is  not  quite  so  simple  as  this,  for  plenty  of 
facts  show  that  adhesion  or  solution  relations  between  dissolved 
substance  and  the  membrane  play  also  a  part.  Moreover,  the 


How  Plants  Draw  in  Various  Materials 


179 


condition  of  balance  in  a  saturated  membrane  explains  how  it 
is  that  water  can  pass  so  readily  through  it;  for  the  last  films 
absorbed,  those  farthest  from  the  micellae,  are  held  so  very 
lightly  that  only  a  slight  force  is  required  to  draw  them  from  the 
membrane.  What  the  nature  of  the 
force  may  be  which  withdraws  the 
water  from  the  inner  face  of  the  mem- 
brane in  osmosis  we  shall  consider  in 
a  moment. 

Diffusion,  imbibition,  osmosis  it- 
self are  typical  examples  of  molec- 
ular forces,  those  operating  between 
individual  molecules,  in  contrast  with 
the  more  familiar  molar  forces  which 
act  upon  masses.  There  is  also  one 
other  molecular  force  of  some  im- 
portance in  the  plant, — viz.,  capil- 
larity, which  we  must  now  briefly 
notice.  Capillarity  is  the  well- 
known  force  by  which  water  is  raised 
in  small  tubes, — or  any  small  pas- 

<sao-P«  nn  matter  hnw  irrpoiilnr        nnrl     FlG'  60-— A  diagram  to  illustrate  the 

sages  no  matt<  r  now  irregular,  ana  rise  and  depression  of  liquids  in 
the  higher  the  finer  the  tubes,  as  our 
diagram  illustrates  (figure  60).  It 
is  the  power  by  which  a  towel  dries 
water  from  the  skin,  a  blotter  takes  up  ink,  a  wick  raises 
oil,  or  any  porous  substance  soaks  up  liquids.  It  is  only  with 
difficulty,  and  under  suasion  from  my  critic,  that  I  forbear  to 
explain  this  interesting  process  in  detail  to  the  reader;  and  I 
must  regretfully  confine  my  exposition  to  the  following  brief 
synopsis.  The  capillary  rise  of  water  is  due  to  forces  residing 
within  the  water  itself.  Because  the  attractions  mutually  exerted 
between  the  molecules  inside  of  the  liquid  are  not  balanced  at 
the  surface  by  equivalent  attractions  towards  the  outside  (fig- 


capillary  tubes,  drawn  to  approxi- 
mately true  scale.  The  liquid  on 
the  left  is  mercury,  and  on  the  right 
is  water. 


i8o 


The  Living  Plant 


ure  61,  6),  the  surface  layers  of  molecules  are  drawn  strongly 
inward  so  that  collectively  they  press  on  the  liquid  as  if  they  were 
tightly  stretched  rubber, — a  phenomenon  known  as  surface 
tension.  Now  surfaces  that  are  flat  press  inward  with  a  definite 
force,  but  those  which  are  concave,  being  partially  buried,  as  it 
were  (figure  61,  c),  within  the  body  of  the  liquid,  and  therefore 
having  the  inward  attractions  of  the  molecules  a  little  com- 


a  b  c 

FIG.  01.- -A  diagram  to  illustrate  the  operation  of  forces  concerned  in  capillarity,  repre- 
senting sections  through  convex,  flat,  and  concave  water  surfaces.  The  small  circles, 
open  and  solid,  are  water  molecules,  and  the  larger  circles  are  the  areas  within  which 
given  molecules,  represented  black,  are  cohesively  attracted  by  others.  Where  these 
areas  lie  wholly  within  the  liquid,  as  shown  in  the  lower  part  of  6,  the  attractions 
balance  one  another,  and  no  effect  is  produced;  but  where  the  areas  fall  partly  outside 
of  the  liquid,  the  inward  attractions  are  not  resisted  by  equivalent  outward  ones, 
though  the  exact  degree  thereof  depends  on  the  form  of  the  surface. 

pensated  by  partial  attractions  outward,  press  inwards  with  less 
force,  while  those  which  are  convex,  projecting  as  it  were  outside 
of  the  liquid,  have  their  molecules  drawn  in  with  an  even  stronger 
attraction  than  have  those  of  a  flat  surface  (figure  61,  a).  There- 
fore it  follows  that  the  very  mobile  water  will  always  be  pressed 
away  from  flat  or  convex  surfaces  towards  those  which  are  con- 
cave. Now  it  happens,  furthermore,  that  water  adheres  both  to 
glass  and  to  wood,  and  hence  in  a  tube  of  either  of  these  substances 


How  Plants  Draw  in  Various  Materials  181 

climbs  up  a  bit  on  the  wall,  as  our  figure  61,  c  well  illustrates,  mak- 
ing the  surface  concave  to  a  degree  that  is  greater  the  smaller  the 
tube.  Hence  the  greater  surface  tension  of  the  flat  surface  outside 
pushes  the  mobile  water  up  against  the  lesser  pressure  of  the  con- 
cave surface  inside,  forcing  it  to  rise  against  gravitation  until  equi- 
librium is  established,  which  will  occur  at  a  higher  point  the  smaller 
the  tube.  And  the  reverse  process  occurs  with  liquids  which 
will  not  adhere  to  glass  or  wood,  e.  g.,  mercury,  or  with  walls 
of  such  composition  that  water  will  not  adhere  thereto,  as  in 
some  air  passages  of  plants;  for  in  this  case  the  surface  in  the 
tube  is  convex,  and  presses  the  water  down  against  the  flat  sur- 
faces outside,  so  that  the  liquid  stands  below  the  outside  level  in 
the  tube  (figure  60,  on  the  left),  or,  if  the  tube  is  not  deeply  im- 
mersed, will  not  enter  at  all. 

Such  is  capillarity,  deriving  its  energy  from  internal  molecular 
tensions  given  release  by  peculiarities  of  external  conditions,  and, 
like  all  molecular  forces,  strictly  limited  in  amount  and  without 
possibility  of  continuous  action.  Capillarity  plays  in  the  plant 
some  minor  part  in  the  ascent  of  sap,  in  prevention  of  the  entrance 
of  water  into  some  air  passages,  and  in  other  processes  later  to  be 
noted.  Moreover,  some  physicists  see  in  imbibition  nothing  but 
a  refined  capillarity,  although  as  I  think,  the  phenomena  of  im- 
bibition of  water  vapor,  presently  to  be  noted  under  hygrosco- 
picity,  is  hardly  consonant  with  this  explanation.  Still  another 
possible  connection  of  a  refined  capillarity  with  osmotic  absorp- 
tion will  be  noticed  in  a  moment. 

We  have  now  reached  the  place  where  the  reader  who  may 
have  used  my  permission  to  skip  for  a  little  must  resume  his 
grasp  on  this  narrative  if  he  is  to  understand  the  essentials  of 
osmotic  phenomena. 

In  watching  the  ascent  of  a  liquid  in  an  osmoscope,  like  that 
of  figure  56,  one  sooner  or  later  comes  to  wonder  what  would 
happen  in  case  an  insuperable  barrier,  e.  g.,  a  tight  stopper,  were 
interposed  against  the  further  rise  of  the  liquid.  The  matter  is 


182  The  Living  Plant 

easy  of  experiment  and  the  answer  plain; 
the  cup  becomes  stretched  or  even  pushed 
from  the  tube,  or  sometimes  (and  always  if 
provided  with  a  semipermeable  membrane) 
it  bursts.  This  shows  that  osmotic  ab- 
sorption, if  confined,  develops  osmotic 
pressure.  Of  course  the  pressures  have 
been  measured  exactly,  chiefly  by  aid  of  an 
instrument  invented  by  the  great  botanist 
Pfeffer,  and  shown  by  the  accompanying 
picture  (figure  62).  When  its  porous  cup, 
lined  with  a  semipermeable  membrane,  is 
filled  with  a  solution  of  sugar  like  that  in 
root  hairs,  and  then  is  immersed  in  pure 
water,  the  gauge  actually  exhibits  a  pres- 
sure equal  to  that  of  three  or  four  atmos- 

CQ>    i  I!    pheres,  or  fifty  to  sixty  pounds  to  the  square 

I    J)          PJI]    inch.    Nor  is  this  all,  for  when  very  strong 

solutions  are  used,  which  require,  of  course, 

FIG  62  —  Pfcffcr's  ceil  as  an  instrument  of  enormously  greater 
strength,  pressures  of  surprisingly  high 


to  M  his  size).  magnitude  have  been  registered,  even  up  to 

A      semipermeable    mem- 

brane is  formed  ail  over  twenty-four  atmospheres,  or  360  pounds  to 

the    inner    face    of    the    ,  ,  ,  i     i_  •    T. 

porcelain  cup,  which  is  the  square  inch,  —  a  much  higher  pressure 

Bright  ollhe^ur"!    lndeed  than  6Ver  1S  US6d  ln  the  Steam  b°ilerS 

The  cup,  and  ail  the  re-  of  even  the  swiftest  express  locomotives; 

mainder  of  the  appara- 

tus,  is  then  filled  with  while  recently  even  higher  ones  have  been 
which8,"  Absorbing  water  measured.  Nor  are  such  pressures  of  merely 
when  the  cup  is  im-  academic  interest  to  the  botanist,  because 

mersed,  presses  the  mer- 

cury up  against  the  air  others  higher  yet,  above  one  hundred   at- 

in  the  gauge  to  a  height  •          j    «.  •  x          j 

which    balances,     and  mospheres,  have  been  found  to  exist  under 

The^maining  meThant    Special  Conditions  in  plant  Cells. 

cai   features  are    con-       jjere  fonows  another  paragraph  which  the 

nected  with   filling   and  F         °      J 

sealing  the  cup.  reader  may  skip  if  such  be  his  inclination, 


How  Plants  Draw  in  Various  Materials  183 

since  it  merely  concerns  the  explanation  of  osmotic  pressure, 
and  is  not  essential  to  the  integrity  of  our  subject.  Now  a 
very  remarkable  and  important  point  about  osmotic  pressures 
is  this,  that  in  general  they  are  the  same  in  amount  as  would 
be  given  by  the  respective  substances  if  converted  into  gases 
at  the  same  volume,  temperature  and  pressure.  This  carries 
the  implication  that  osmotic  pressures  and  gas  pressures,  be- 
ing the  same  in  amount,  are  the  same  in  kind,  the  dis- 
solved substance  being  practically  a  gas,  and  it,  not  the  liquid, 
exerting  the  pressure.  But  while  this  explanation  is  satisfactory 
for  most  of  the  phenomena,  it  meets  with  the  physical  difficulty 
that  the  closely  packed  water  molecules  must  prevent  that  free- 
dom of  back  and  forth  movement  upon  which  a  gas  pressure 
depends.  Accordingly  a  second  explanation  has  been  given, 
really  an  old  one  revived,  which  finds  the  source  of  the  pressure 
in  an  adhesive  attraction  between  the  molecules  of  the  dissolved 
substance  and  those  of  the  water,  whereby  the  former  draw  all 
of  the  latter  around  them,  and  take  more  from  the  membrane 
(which  easily  recoups  itself  from  the  outside  supply) ;  and  thus 
the  solution  swells  and  the  pressure  is  obviously  exerted  by  the 
substance  and  liquid  in  combination.  Or,  one  can  express  the 
same  thing  by  imagining  that  the  molecules  of  the  dissolved 
substance  act  like  the  micella  of  the  membrane  and  absorb 
water  (from  the  latter)  by  imbibition,  with  only  this  difference 
that  the  adhesion  between  substance  and  water  is  stronger  for 
all  distances  than  the  cohesion  of  the  substance  for  itself.  And 
still  a  third  explanation  is  possible,  namely,  that  the  spaces 
between  the  suspended  molecules  of  the  dissolved  substance  act 
like  excessively  fine  passages  along  which  the  water  passes  forcibly 
by  an  extremely  refined  capillarity, — in  which  case  the  water,  and 
not  the  substance,  exerts  the  pressure.  And  if  it  seems  that 
the  correspondence  between  osmotic  pressures  and  gas  pressures 
must  be  conclusive  for  the  first  explanation  against  the  others,  it 
is  to  be  said  that  this  is  not  necessarily  true,  for  the  properties 


1 84 


The  Living  Plant 


of  substances  in  solution  and  in  the  gaseous  state  are  so  closely 
and  regularly  interconnected,  that  the  same  mathematical  rela- 
tions apply  to  them  all.  And  as  to  which  of  the  explanations  is 
correct,  the  future  must  decide. 

The  reader  has  now  a  sufficiency  of  data  for  understanding 
pretty  fully  the  nature  of  osmotic  absorption  and  pressure,  which 
we  may  summarize  here  by  aid  of  the  accompanying  diagram 

(figure  63).  The  dissolved  sub- 
stance inside  of  a  membrane  is 
always  tending  to  diffuse  out- 
ward by  the  energy  of  its  own 
diffusion  pressure,  which  de- 
pends ultimately  upon  heat; 
and  if  the  membrane  be  per- 
meable, then  the  substance  dif- 
fuses into  and  beyond  it,  as  it 
did  from  our  molasses-holding 
osmoscope;  but  if  semiperme- 
able  then  not.  Meantime, 
whether  because  the  interrupted 

FIG     63.-Diagram    to    illustrate    osmosis  diffusion-preSSUTC  acts  like  gas- 
through    a    permeable    membrane;    the 

symbols  as  in  figures  58,  59.     In  case,  prCSSUTC    to    Swell    the     interior 
however,   the   membrane   is   semiperme-  .  .  ,  .         ,, 

able  the  dissolved  substance  cannot  es-  liquid,    Or    DeCailSC    OI     adhCSlVC 

cape  through  it.  attraction    between    substance 

and  liquid,  or  because  of  capillary  action  between  substance 
and  liquid,  the  substance  draws  on  the  water  supplied  by  the 
membrane,  which  yields  it  very  easily  so  long  as  it  can  recoup 
itself  freely  from  the  outside  supply.  Thus  the  solution  swells 
and  exerts  pressure  until  the  power  of  the  substance  to  with- 
draw water  from  the  membrane  exactly  balances  the  resistance 
interposed  to  its  expansion.  And  this  is  all  true  inside  of  the 
plant  or  the  animal  as  well  as  outside  thereof,  whence  we  may 
now  deduce  another  of  our  natural  verities,  to  this  effect, — that 
wherever  the  conditions  for  osmotic  absorption  exist,  the  membrane 


How  Plants  Draw  in  Various  Materials  185 

acts  as  a  check,  either  partial  or  total,  to  the  further  diffusion  of  the 
dissolved  substance  while  allowing  the  liquid  itself  to  pass  freely,  as 
a  result  of  which  the  dissolved  substance,  whether  by  gas-like  ex- 
pansion or  direct  attraction,  draws  liquid  through  the  membrane, 
swells,  and  exerts  an  osmotic  pressure  proportional  to  its  strength. 

After  this  lengthy  but  needful  discussion  of  physical  principles, 
we  turn  to  the  actual  osmotic  phenomena  displayed  by  plants, 
and  here  the  reader  who  is  skipping  the  hard  parts  must  resume 
the  narrative.  The  absorption  of  water  by  roots  is  the  most 
important  of  these  phenomena,  but  there  are  others  of  little  less 
consequence.  First  among  them  is  the  maintenance  of  rigidity 
in  very  soft  parts  such  as  leaves,  young  stems  and  flowers.  These 
parts  consist  mostly  of  water  (fully  90  per  cent),  while  the  re- 
siduum of  solid  matter  (about  10  per  cent)  is  too  small  and  un- 
substantial to  supply  rigid  support.  Even  the  moderately  firm 
veins,  as  everyone  knows,  are  quite  unable  to  keep  a  wilted 
leaf  from  collapsing.  But  every  young  cell,  soft  and  weak 
though  it  is,  can  absorb  water  powerfully  through  its  semi- 
permeable  protoplasmic  membrane  into  its  sugar-holding  sap, 
and  thus  swell  to  turgescence,  stretching  the  walls  until  they  are 
tense,  and  the  structure  is  stiff.  Again,  osmotic  pressure  supplies 
the  energy  by  which  young  cells  can  expand  their  walls  in  growth, 
overcoming  the  resistance  of  older  cells  around  them;  by  which 
buds  or  flowers  can  swell  and  unfold;  by  which  young  roots  can 
force  a  way  through  hard  soil  and  even  destroy  masonry  and  lift 
curbstones ;  and  by  which  soft-bodied  fungi  can  burst  pavements. 
Osmotic  pressure  is  the  mechanical  power  used  by  those  parts  in 
effecting  their  work. 

Of  minor  osmotic  phenomena  in  plants,  some  of  them  familiar 
in  the  household,  there  are  many.  Thus,  if  one  places  dry  sugar 
on  fresh  strawberries,  pretty  soon  it  becomes  a  syrup,  and  the 
berries  look  shrunken;  evidently  the  sugar,  moistened  by  con- 
tact with  the  berry,  makes  a  dense  solution  which  draws  water 
from  the  cells.  The  collapse  of  berries  from  this  cause  is  very 


n 
( 


1  86  The  Living  Plant 

evident  when  preserves  are  made  with  plenty  of  sugar,  but 
fruits  retain  their  shape  in  some  of  the  processes  where  little  or 
no  sugar  is  used.  Dry  raisins  and  currants  become  plump  when 
soaked,  for  their  cells  contain  sugar  though  their  protoplasm  is 
dead;  and  the  process  is  hastened  by  the  heat  of  cooking.  The 
crisping  of  celery  or  cucumbers  when  placed  in  water  is  a  case  of 
increased  turgescence,  the  tense  cells  actually  exploding,  as  it 
were,  when  crushed  by  the  teeth.  The  reason,  by  the  way,  why 
the  water  must  be  cold  for  best  crisping  is  this,  that  warmer 
water  tends  to  drive  out  and  replace  the  air  of  the  intercellular 
passages,  thus  deadening  the  explosive  action  in  which  crisping 
consists.  Moreover,  the  bursting  of  hard-skinned  berries,  like 
cranberries,  when  heated  in  water,  though  apparently  an  os- 
motic phenomenon,  is  primarily  due  to  the  swelling  of  the  air 
confined  by  the  skin,  the  same  thing  which  occurs  in  apples  when 
baking.  A  genuine  osmotic  bursting  does,  however,  occur  some- 
times in  fruits,  like  plums  and  grapes,  while  still  on  the  plant, 
because  of  a  great  absorption  of  water  from  the  ducts  by  the 
sugar-ripe  cells  under  action  of  heat  on  bright  summer  days; 
and  the  calyx  of  carnations  sometimes  bursts  from  the  same 
cause  when  the  temperature  rises  in  the  greenhouse.  There  is 
in  tomatoes  an  osmotic  disease,  called  (Edema,  due  to  an  over- 
absorption  of  water  by  soft  cells,  and  the  consequent  formation 
of  blistery  swellings.  The  swelling  of  soaking  seeds  with  a  power 
sufficiently  great  to  result  in  the  bursting  of  strong  vessels,  is 
chiefly  due  to  osmosis  though  it  is  partly  imbibition,  and  the 
same  is  true  of  the  forcible  swelling  of  dried  apples.  Sugar  and 
salt  are  common  preservatives,  the  one  of  fruits  and  the  other  of 
meats,  though  neither  is  really  poisonous  to  the  germs  and  molds 
which  cause  decay,  while  the  former  is  actually  nutritive;  but  in 
strong  solutions  they  act  germicidally,  because  they  withdraw 
so  much  water  from  the  decay  organisms  as  to  render  these 
inactive.  Moreover,  either  of  these  substances,  when  eaten  in 
more  than  moderate  amount,  causes  thirst,  which  results  from 


How  Plants  Draw  in  Various  Materials  187 

their  osmotic  action  in  withdrawing  water  from  the  walls  of  the 
stomach,  whose  dryness,  from  whatsoever  cause,  gives  the  thirst 
sensation.  And  there  are  doubtless  other  familiar  osmotic 
phenomena  which  will  occur  to  the  ingenious  reader,  who  can 
now  have  the  pleasure  of  undertaking  their  explanation  upon  an 
osmotic  basis. 

To  complete  our  discussion  of  water  absorption  by  plants,  we 
must  consider  the  case  of  dry  tissues  like  wood.  Dry  wood,  as 
everyone  knows,  absorbs  water  eagerly  and  powerfully,  swelling 
considerably  in  the  action.  The  conditions  for  osmosis  are  ab- 
sent, and  all  evidence  goes  to  show  that  the  absorption  is  due  to 
imbibition  into  the  solid  cell-walls.  This  helps  to  explain  a 
common  phenomenon  in  connection  with  wood, — its  warping. 
When  water  is  placed  on  one  side  of  a  dry  board,  the  board  warps 
away  from  the  wet  side,  often  with  power  enough  to  tear  it  from 
firmly-fixed  fastenings;  but  if  the  supply  of  water  be  continued, 
the  board  later  flattens  out,  and  a  measurement  will  show  that  the 
saturated  board  is  considerably  larger  than  when  dry,  precisely 
as  a  membrane  is.  Evidently,  the  water  forcibly  absorbed  by 
imbibition  upon  one  side  forces  apart  the  micellae  and  swells 
the  wood  on  that  side  before  it  has  time  to  reach  the  other,  al- 
though, after  the  lapse  of  enough  time,  it  penetrates  to  the  other 
side,  swells  that,  and  thus  straightens  the  board,  as  represented 
diagrammatically  by  a  combination  of  the  figures  59  and  64.  It 
will  here  occur  to  the  reader,  incidentally,  that  boards  often  warp 
without  access  to  water,  and  simply  from  the  one-sided  action  of 
heat.  The  principle,  nevertheless,  is  the  same;  even  the  dryest 
boards  contain  some  water,  the  drying  of  which  from  one  side 
allows  the  water  remaining  in  the  other  to  warp  the  board  in  the 
usual  manner.  Furthermore,  the  reader  may  recall  that  a  board 
will  warp  crosswise  but  never  lengthwise,  which  fact  is  correlated, 
obviously,  with  another  well-known  fact  about  wood, — a  fact 
of  very  great  importance  in  building  and  carpentry, — viz.,  that 
wood  does  not  lengthen  or  shrink  lengthwise  as  it  does  so  freely 


1 88  The  Living  Plant 

crosswise.  The  basis  of  this  fact  is  not  known,  but  I  venture  to 
suggest  as  a  possible  explanation  that  the  sides  of  the  cubical 
micellae  facing  towards  the  end  of  the  wood  (those  towards  and 
away  from  the  reader  in  the  sections  of  figures  59  and  64)  have 
no  attraction  for  water  at  all,  and  hence  absorb  none;  and  this 
view  I  propose  that  we  hold  as  an  hypothesis  until  it  is  disproven 


FIG.  64. — A  diagram  illustrating  the  molecular  basis  of  the  warping  of  wood.    It  belongs 
between  a  and  b  of  figure  59. 

or  a  better  is  offered.  The  supposition  that  micellar  surfaces  can 
exist  without  any  attraction  for  water  will  help  also  to  explain 
how  cell-walls  can  be  waterproof,  as  they  actually  are  in  cork 
and  epidermis. 

A  special  form  of  imbibition  by  dry  tissues  is  the  absorption 
of  water  vapor  from  moist  air,  with  its  return  thereto  as  the 
air  becomes  dry, — a  phenomenon  called  hygroscopicity.  Fa- 
miliar examples  occur  in  the  softening  and  sagging  of  paper  in 
damp  weather,  in  the  uncurling  and  curling  of  hair,  in  the  move- 
ments of  the  wood  of  old  furniture,  giving  rise  to  snappings  and 
creakings  which  are  oft  of  uncanny  effect  when  heard  in  the 
stillness  of  night.  Now,  in  essence,  hygroscopic  movement  is  the 
same  thing  as  warping,  the  water  being  absorbed  as  a  vapor 
instead  of  as  liquid.  Furthermore,  if  the  tissues  are  made  very 


How  Plants  Draw  in  Various  Materials  189 

thin,  this  warping  may  be  rapid  enough  to  be  seen  by  the  eye, 
and  forcible  enough  to  exert  a  considerable  pressure;  and  ad- 
vantage of  these  features  is  taken  by  plants,  to  produce,  by  aid 
of  suitable  mechanical  arrangements,  adaptive  movements  of 
various  sorts.  Of  this  nature  are  sundry  hygroscopic  movements 
described  elsewhere  in  this  book, — the  self-planting  of  some 
seeds;  the  creeping  of  some  fruits  by  the  twisting  movements 
of  hygroscopic  awns;  the  opening  and  closing,  with  changes  of 
weather,  of  most  spore-cases  and  anthers;  and  the  forcible  shoot- 
ing of  seeds  by  hygroscopically-bursting  pods.  Man  has  also 
taken  advantage  of  this  principle  to  construct  instruments, 
called  hygroscopes  or  hygrometers,  for  showing  or  measuring 
the  amount  of  moisture  contained  in  the  air.  By  suitable  mechani- 
cal arrangements  the  hygroscopically  swelling  or  shrinking  sub- 
stance may  be  made  to  twist  a  pointer  over  a  graduated  scale, 
to  cause  suitably-clad  little  persons  to  make  their  exits  and  en- 
trances to  and  from  tiny  houses,  or  to  produce  other  visible 
results  having  appropriate  significance. 

So  much  for  the  absorption  of  water;  we  turn  now  to  absorp- 
tion of  minerals,  several  kinds  of  which  are  needed  for  the  various 
processes  of  metabolism  inside  of  the  plant.  But  the  subject  is 
comparatively  simple.  The  plant  can  absorb  only  those  minerals 
which  exist  in  solution  in  the  water  of  the  soil,  dissolved  therein 
from  the  rocks  or  from  various  fertilizers  added  by  man.  And. 
the  minerals  enter  the  plant  with  the  water.  In  Water-plants, 
and  the  simpler  sorts  of  the  land,  they  enter  mostly  by  diffusion 
from  the  outside  supply,  traveling  everywhere  through  the  water 
which  saturates  the  plant.  But  in  the  higher  plants  they  are 
swept  in  with  the  current  through  the  hairs,  cortex  and  ducts, 
from  which  they  pass  by  diffusion  to  the  places  of  use.  It  would 
seem  at  first  sight  that  their  passage  through  hairs  and  cortex 
would  be  forbidden  by  the  semi-permeable  protoplasmic  mem- 
branes. But  semi-permeability  is  wholly  relative,  and  a  given 
membrane  which  prevents  the  passage  of  the  relatively  large 


The  Living  Plant 


sugar  molecules,  may  permit  the  passage  of  the  much  smaller 
mineral  molecules.  But  aside  from  this,  the  evidence  shows  that 
in  protoplasmic  membranes  another  influence  comes  into  play, 
and  that  the  dissolved  substance,  in  order  to  pass  through  such 
a  membrane,  must  be  soluble  in  the  material  composing  it. 

There  remains  to  be  considered  the  absorption  of  gases,  a  matter 
of  great  importance  because  of  the  indispensable  part  played  in  the 

plant's  economy  by  both  car- 
bon dioxide  and  oxygen,  the 
great  reservoir  of  which  is 
the  air.  The  first  requisite, 
of  course,  to  gas  absorption 
by  the  living  cells,  the  most 
of  which  he  deeply  buried 
within  the  body  of  the  plant, 
is  some  system  whereby  those 
gases  can  be  conveyed  from 
the  atmosphere  into  their 
presence;  and  such  a  sys- 
tem, as  the  reader  already 

FIG.  05.— A  cluster  of  cells  in  a  piece  of  pith,    v         1pornprl   in   Phnnrpi-  TT    i«? 
showing    the    intercellular    air    passages    (in  naS  learnecl  m  <^naple      11,  IS 

black).    (Copied  from  a  wail  diagram  by  provided  in  the  inter-cellular 

Frank  and  Tschirch.) 

air  passages,  which  are  shown 

in  a  typical  tissue,  a  bit  of  pith,  in  the  accompanying  picture  (fig- 
ure 65).  These  passages  do  not  exist  in  young  tissue  where  new 
cells  are  in  process  of  formation,  as  figures  53  and  139  C  illustrate; 
but  as  the  young  cubical  cells  grow  larger,  they  tend  to  round  off 
into  spherical  form,  splitting  in  their  mid-walls,  first  at  the 
angles  and  then  along  the  edges,  until  the  final  arrangement 
tends  to  approximate  to  that  of  the  spaces  and  passages  existing 
between  balls  in  a  pile.  These  passages  once  formed  always 
persist,  no  matter  what  shapes  the  cells  may  assume;  and  there- 
fore they  form  a  continuous  system  ramifying  everywhere 
throughout  the  plant,  as  is  represented  diagrammatically  in  the 


How  Plants  Draw  in  Various  Materials  191 

accompanying  figure  66,  A.  Here,  for  simplicity,  the  passages, 
represented  in  black,  are  imagined  to  fall  into  one  plane;  and  here 
also,  by  the  way,  a  partial  interruption  in  the  system  due  to 
the  presence  of  the  longitudinally-running  woody  bundles  is 
shown  by  the  blank  spaces.  In  young  green  tissues,  as  shown  by 
the  detailed  diagram  (figure  66,  B),  in  which,  as  in  C  and  D,  the  air 
passages  are  partially  reduced  to  one  plane,  the  passages  open 
through  the  epidermis  by  the  stomata,  while  on  older  stems, 
where  a  corky  bark  has  formed,  they  open  through  the  lenticels 
(figure  66,  C),  those  corky  wart-like  excrescences  prominent  on  all 
young  stems,  and  consisting  simply  of  open  gashes  in  the  bark, 
partially  sealed  in  the  winter  by  corky  cortical  cells.  In  young 
roots,  however  (figure  66,  D),  neither  stomata  nor  lenticels  are 
present,  but  the  continuous  epidermis  and  hairs  are  commonly 
and  normally  covered  with  films  of  water,  through  which  the 
gases  diffuse  in  solution  from  the  air  spaces  in  the  soil  to  those  in 
the  root,  and  vice  versa. 

Thus  much  for  the  aeration  system,  whereby  every  living  cell 
of  the  plant  is  brought  into  communication  with  the  external 
reservoir  the  air.  But  what  is  the  power  impelling  the  gases 
along  these  passages, — which  are  often  of  great  length,  small  size, 
and  extreme  irregularity?  Plants  possess  no  mechanism  for  the 
forcible  indrawing  and  expulsion  of  the  air  en  masse,  such  as 
animals  have  developed  in  their  muscular  chest-and-lung  breath- 
ing arrangements.  In  some  degree  a  movement  of  air  through 
the  inter-cellular  system  is  promoted  by  the  swaying  of  parts  in 
the  wind,  and  by  the  expansions  and  contractions  of  the  air  under 
varying  temperature  and  barometric  pressure;  but  such  effects 
are  insignificant.  The  primary  cause  of  the  gas  movement  is 
found  in  diffusion,  that  process,  already  described,  whereby  the 
molecules,  driven  by  the  energy  of  heat  absorbed  from  the  sur- 
roundings, tend  ever  to  move  forward  from  places  of  greater  to 
places  of  lesser  concentration,  and  therefore  from  places  where 
they  are  being  formed  or  released  to  places  where  they  are  not, 


VJ&JT?'  Longitudinal  section  through  half  of  stem  at  y. 


Longitudinal  section  through  a  portion  of  root  at  * 


FIG.  66.— Generalized  drawings  illustrating  the  aSration  system  of  the  plant. 
192 


How  Plants  Draw  in  Various  Materials  193 

and  from  places  where  they  occur  to  places  where  they  are  being 
absorbed.  Moreover,  each  kind  of  gas  diffuses  by  itself,  no  mat- 
ter what  others  may  be  present,  so  that  a  gas  in  process  of  ab- 
sorption by  a  plant  can  move  inward  in  a  steady  stream  through 
another  which  is  not  being  absorbed,  and  even  against  the  op- 
posite stream  of  one  in  process  of  release.  Thus,  in  photosynthe- 
sis, for  example,  a  constant  current  of  carbon  dioxide  diffuses 
into  the  leaf,  through  nitrogen  which  remains  without  move- 
ment, against  a  current  of  oxygen  which  is  diffusing  outward.  It 
is  a  condition  hard  to  imagine,  it  is  true,  but  the  facts  declare  it 
is  so.  The  gases  thus  impelled  along  the  passages  by  diffusion 
finally  reach  the  living  cells,  and,  being  soluble  in  water,  are 
dissolved  by  the  moist  surfaces,  and  then  diffuse  through  walls 
and  protoplasm  to  the  places  of  use.  And  here  I  may  add  a  sug- 
gestion, for  the  benefit  of  the  reader  versed  in  physics,  that  this 
movement  of  different  gases  in  contrary  directions  along  the  same 
passages  is  explained  much  better  by  the  old-fashioned  idea  that 
diffusion  and  gas  pressure  are  caused  by  a  mutual  repulsion 
between  the  same  kind  of  molecules  than  by  the  modern  kinetic 
theory,  which  makes  those  phenomena  the  result  of  vibratory 
movements  of  the  molecules;  and  moreover  the  very  same  con- 
ception explains  perfectly  how  osmotic  pressures  and  gas  pres- 
sures can  be  identical  in  kind  as  well  as  in  quantity. 

A  very  special  case  of  absorption  occurs  in  those  plants  which 
absorb  organic  food  substances  already  made.  Such  plants,  of 
which  parasites  are  a  good  example,  have  the  power  of  excreting 
from  their  absorbing  parts  those  special  enzymes,  or  ferments, 
which  render  soluble  the  organic  materials  they  touch.  The 
dissolved  substance  then  enters  the  plant  by  diffusion  from  the 
place  of  high  concentration  outside  to  the  places  of  use  and  low 
concentration  inside, — the  intermicellar  spaces,  of  course,  being 
adjusted  for  the  admission  of  these  large  molecules.  The  ab- 
sorption by  pollen-tubes  of  tissues  through  which  they  pass;  of 
humus  by  the  fungi  which  live  thereupon;  and  of  the  materials 


i94  The  Living  Plant 

dissolved  from  the  bodies  of  insects  by  the  pitcher-plants  or  other 
insectivora,  is  also  of  this  character.  In  all  of  these  cases  the 
materials  are  not  drawn  in,  as  by  osmosis,  but  are  driven  in  by 
the  energy  of  their  own  diffusion. 

In  reviewing  absorption  by  plants,  the  reader  must  be  struck 
by  the  fact  that  the  forces  at  work  are  chiefly  molecular,  and 
therefore  slow  and  gradual,  even  though  powerful  in  their  action. 
Plants,  as  it  were,  arrange  the  conditions  to  permit  the  molec- 
ular forces  to  work  for  them.  In  this  respect  they  stand  in 
rather  marked  contrast  to  animals,  which  tend  rather  to  make 
use  of  those  larger  or  molar  forces  which  permit  greater  rapidity 
and  range  of  action.  In  this  difference  we  have  the  explanation 
of  the  persistent  placidity  of  plants  in  comparison  with  the 
abounding  activity  of  animals. 

This  chapter  is  already  so  long  that  it  is  only  with  reluctance 
that  I  add  anything  more;  but  there  remain  a  few  matters  which 
must  receive  some  discussion  in  this  immediate  connection.  First, 
we  must  examine  a  little  farther  the  arrangements  for  aeration  in 
plants,  especially  under  unusual  conditions.  Wherever  particular 
need  exists,  there  the  inter-cellular  system  may  become  much 
larger,  as  occurs  conspicuously  in  leaves,  which,  requiring  a 
carbon  dioxide  supply  for  photosynthesis  ten  times  or  more 
greater  than  the  oxygen  supply  they  need  for  respiration,  exhibit 
a  far  larger  aeration  system  than  any  part  of  the  plant  needing 
only  a  respiration  supply;  and  that  is  why  leaves  have  the  mark- 
edly spongy  texture  they  so  commonly  exhibit.  Again,  there  are 
plants  of  such  habit  that  their  roots  (as  in  Marsh  Plants),  or 
even  huge  rootstocks  (as  in  Water  Lilies),  lie  deep  under  water 
and  must  be  aerated  in  some  way  from  the  surface.  In  such  cases 
the  inter-cellular  system  is  immensely  developed,  even  to  the 
formation  of  elaborate  passages,  in  the  parts  which  lead  from  the 
surface  to  the  parts  under  water;  and  this  is  the  reason  for  the 
soft,  open,  spongy  texture  of  the  petioles  of  Water  Plants,  and 
the  pith  of  Rushes  and  Sedges,  and  it  explains  why  some  plants 


How  Plants  Draw  in  Various  Materials  195 

can  grow  in  a  soil  that  has  no  aeration.  And  it  is  interesting  to 
note,  by  the  way,  that  many  interesting  accessory  adaptions 
are  displayed  by  these  plants,  of  which  one  in  particular  is  here 
apposite,  viz.,  the  walls  of  the  air  passages  in  these  Water  Plants 
are  so  modified  chemically  that  water  will  not  wet  them,  and 
therefore  will  not  enter  them  by  capillarity,  on  the  principle  dis- 
cussed earlier  in  this  chapter.  This  is  obviously  an  advanta- 
geous adaptation  against  the  obstruction  of  these  slender  passages 
by  water  in  case  of  sub-aqueous  accident  to  the  petioles  or  stems. 
In  some  other  cases  accessory  aeration  structures  are  developed 
which  permit  a  shorter  route  from  the  air  to  the  roots.  Of  this  a 
conspicuous  case  has  been  claimed  to  exist  in  the  great  knees  of 
the  Bald  Cypress  of  the  Southern  swamps,  which  rise  above  the 
water  surface  and  contain  an  aeration  system  in  connection  with 
the  roots;  and  other  comparable  cases  are  known.  In  some 
Water  Plants,  however,  the  aeration  is  of  a  simpler  sort,  con- 
sisting indeed  of  an  absorption  of  air  dissolved  in  the  water,  in 
precisely  the  manner  used  by  the  Fishes.  In  some  kinds,  for 
example  some  Eel-grasses,  the  leaves  are  so  thin  as  to  present 
a  relatively  great  surface  in  proportion  to  the  bulk  of  tissue  to 
be  ae'rated;  while  in  others  the  leaves  are  cut  to  the  finest  divi- 
sions, presenting  indeed  a  condition  directly  comparable  physio- 
logically with  the  gills  of  the  fish.  This  is  the  reason  for  the  tissue- 
thin  and  thread-fine  structure  of  practically  all  plants  which  live 
wholly  under  water. 

Finally  we  must  give  some  further  attention  to  the  particular 
organs  of  absorption,  the  Roots.  The  structure  of  the  young  white 
tips  has  already  been  described  except  for  one  point,  viz.,  the 
water-carrying  ducts  and  the  food-carrying  sieve-tubes  do  not 
stand  in-and-out  from  one  another  as  in  young  stems,  but  alter- 
nately. In  this  arrangement  lies  an  obvious  adaptation,  since 
it  removes  the  sieve-tubes  out  of  the  path  of  the  water  from 
hair  cells  to  ducts;  and  this  conclusion  receives  some  confirma- 
tion from  the  further  fact  that  the  arrangement  is  not  main- 


196  The  Living  Plant 

tained  in  the  older  part  of  the  root,  where  the  entire  anatomy  is 
closely  like  that  of  the  stem.  Roots,  however,  have  no  nodes, 
nor  regular  places  of  origin  of  new  roots,  which,  unlike  branches, 
originate  deep  in  the  tissues,  budding  out  as  it  were,  from  the 

fibro-vascular  bundles  (figure  67), 
and  breaking  their  way  (partially 
by  the  aid  of  enzymes)  out 
through  the  cortex,  at  places  de- 
termined by  the  stimulus  of  more 
abundant  air,  water,  minerals,  or 
space.  This  method  of  origin  of 
side  roots,  by  the  way,  stands  in 
marked  contrast  with  that  of  side 
stems,  or  branches,  which  always 
originate  by  a  transformation  of 
the  cells  of  the  cortex,  as  indi- 

Fio.  67.—  A    cross    section  of   a  typical  .  .  , 

root,  showing  the  way  in  which  a  side    Gated    in    figure    137.        Thus,    the 


system  of  any  plant  is  al- 

NatUTe   and  Developmeni  of  ways    excessively    irregular,    al- 
though, on  the  other  hand,  differ- 

ent kinds  of  roots  present  comparatively  little  variation  in 
structure  or  appearance,  as  indeed  is  to  be  expected  from  the 
comparatively  uniform  conditions  under  which  most  of  them  live. 
Typically,  roots  are  much  more  slender  than  stems,  and  have 
their  strengthening  tissues  condensed  nearer  the  center,  in  obvi- 
ous correlation  with  the  fact  that  they  have  no  lateral  strains 
to  withstand,  but  only  pulling  strains  exerted  upon  them  by  the 
stems  for  which  they  must  provide  a  firm  anchorage.  Therefore, 
while  stems  approximate  to  hollow  columns  in  construction,  roots 
approximate  rather  to  ropes  or  cables.  Indeed,  in  many  roots, 
one  can  trace  a  distinction  between  features  connected  with 
absorption  arid  others  connected  with  anchorage  of  the  stems; 
and  the  difference  in  some  cases  goes  so  far  that  one  distinguishes 
between  absorbing  roots  and  anchorage  roots,  which  often  occupy 


How  Plants  Draw  in  Various  Materials  197 

different  positions  or  directions  in  the  soil,  the  former  seeking 
usually  the  dampest  places,  while  the  latter  tend  rather  to  pene- 
trate radiately  from  the  stem  into  the  earth. 

While  absorption  and  anchorage  are  the  typical  functions  of 
roots,  occasionally  they  perform  others  quite  different,  as  we 
have  noticed  already  in  the  chapter  on  leaves  and  stems.  Thus, 
they  become  modified,  with  appropriate  anatomical  changes,  to 
swollen  storage  organs,  in  the  Sweet  Potato;  to  slender  and 
toughened  climbing  organs  in  English  Ivy  and  many  tropical 
climbers;  to  tough  pointed  spines  in  some  Palms;  to  slender 
penetrating  haustoria  or  sucking  organs  in  some  parasites;  to 
flat  green  photosynthetic  organs  in  some  tropical  orchids;  and  to 
yet  other  structures  of  minor  account.  Thus  roots,  like  stems 
and  leaves,  formed  for  one  function  can  be  modified  greatly  for 
the  performance  of  others,  illustrating  once  more  Nature's  won- 
derful capacity  for  ringing  changes  on  her  favorite  ideas. 


CHAPTER  VIII 

THE  WAYS  IN  WHICH  SUBSTANCES  ARE  TRANSPORTED 
THROUGH  PLANTS  AND  FINALLY  REMOVED  THERE- 
FROM. 

Transfer,  Transpiration,  Excretion 

HE  living  plant,  as  the  reader  of  the  foregoing  pages 
will  surely  agree,  can  be  viewed  as  a  kind  of  central 
station  for  the  transformation  of  substance  and  energy, 
both  of  which  forever  are  streaming  into,  passing 
through,  and  issuing  forth  from  the  plant,  undergoing  en  route 
quite  definite  changes  in  correlation  with  adaptive  results.  These 
transformations  we  have  already  considered  in  our  chapters  upon 
Photosynthesis,  Respiration,  and  Metabolism,  while  their  Absorp- 
tion was  the  theme  of  the  chapter  just  finished;  but  we  still  have  to 
consider  their  passage  through  the  plant  and  their  final  removal 
therefrom.  These  matters  can  be  treated  conveniently  together 
as  they  are  in  this  chapter,  although,  for  a  practical  reason  which 
will  later  appear,  we  may  best  reverse  the  natural  order,  and 
treat  first  the  subject  that  logically  should  be  last. 

The  most  abundant  of  the  substances  transferred  and  elimi- 
nated as  well  as  absorbed,  by  plants,  is  water.  Most  people  are 
aware  in  a  general  way  that  plants  are  forever  giving  off  water 
as  vapor  to  the  air,  although  they  have  little  idea  of  its  amount. 
The  fact  can  be  demonstrated,  by  the  way,  very  conclusively  to 
the  eye  by  placing  a  potted  plant,  of  which  pot  and  soil  have 
first  been  enwrapped  by  a  water-tight  covering,  in  a  glass  case 
or  bell-jar,  after  which,  within  a  few  minutes,  there  will  collect 
on  the  glass  a  cloud  of  water-drops  which  can  have  come  from 

198 


How  Substances  are  Transported  and  Removed     199 

no  other  possible  source  than  as  vapor  from  the  leaves.  This  is 
the  source  also  of  most  of  the  moisture  that  collects  upon  win- 
dows near  which  house  plants  are  grown,  and  likewise  of  the 
water-drops  which  gather,  sometimes  to  annoying  extent,  on  the 
glass  faces  of  ferneries,  though  such  water  is  commonly  assumed 
to  originate  from  evaporation  out  of  the  soil.  This  release  of 
vapor  from  leaves  or  other  green  parts  is  a  practically  universal 
phenomenon  in  plants.  It  is  called  in  physiology  Transpiration; 
and  I  wish  to  warn  the  reader  at  this  point,  out  of  the  depths  of 
a  considerable  experience  as  a  teacher,  not  to  allow  a  mere  re- 
semblance in  words  to  create  any  confusion  in  his  mind  between 
this  and  the  utterly  unrelated  process  of  Respiration.  Transpira- 
tion is  one  of  the  great  primal  physiological  facts  about  green 
plants,  and  it  has,  like  Photosynthesis,  this  further  distinction, 
that  it  is  one  of  the  very  few  processes  of  plants  for  which  there 
is  no  equivalent  in  animals,  the  animal  process  of  perspiration 
being  utterly  different  both  as  to  method  and  meaning.  The 
reader  should  therefore  incorporate  into  the  visualized  picture 
of  the  living  plant  now  under  construction  in  his  imagination, 
the  idea  of  a  tenuous  cloud  of  vapor  rising  forever  from  all  its 
green  parts. 

But  no  student  of  science,  and  therefore  I  hope  not  the  reader, 
will  rest  content  with  the  general  fact  that  water  is  given  off  as 
vapor  by  plants,  but  will  insist  upon  knowing  the  quantity.  The 
most  practicable  and  accurate  of  the  several  methods  by  which 
transpiration  quantities  may  be  determined  lies  in  the  use  of  the 
balance.  If  one  takes  an  ordinary  potted  plant, — Fuchsia, 
Hydrangea,  Rubber  Plant,  or  other, — encloses  soil  and  pot  in  a 
water-tight  cover  to  prevent  evaporation  therefrom,  then  weighs 
the  plant  at  intervals  on  an  accurate  balance,  the  comparative 
weights,  aside  from  some  minor,  and  largely  self-compensating, 
errors  arising  from  photosynthesis  and  respiration,  must  obvi- 
ously exhibit  the  exact  transpiration  from  the  leaves  and  the 
stems.  Such  experiments  are  frequently  tried  in  botanical 


200  The  Living  Plant 

laboratories,  and  never  without  exciting  an  interested  attention 
from  all  students,  young  or  old.  Some  of  the  results  are  shown 
vividly  in  the  accompanying  photograph  (figure  68),  wherein  the 
plant,  with  its  pot  and  soil  enclosed  water-tight  for  this  study, 


Fio.  68. — A  potted  Sunflower  prepared  for  transpiration  studies  as  described  in  the  text. 
The  measuring  glasses  show  the  number  of  cubic  centimeters,  and  therefore  of  grams, 
of  water  transpired  in  twenty-four  hours  and  in  a  week.  In  three  and  a  half  days  the 
plant  transpired  a  quantity  of  water  equal  to  the  capacity  of  the  pot  in  which  it  is 
growing. 

is  shown  standing  beside  measuring  glasses  which  display  the  vol- 
ume of  its  transpiration  for  a  day  and  a  week.  The  quantity  of 
transpiration  must  necessarily  depend  on  the  size  of  the  plant ;  and 
in  order  to  compensate  this  variable,  and  at  the  same  time  to 
permit  a  comparison  between  different  plants,  it  is  customary 


How  Substances  are  Transported  and  Removed      201 

to  express   transpiration  in   standard   units.      For  greenhouse 
plants,  which  have  been  the  most  carefully  studied  from  this 
point  of  view,  it  has  been  found  that  while  the  transpiration  in 
one  hour  from  one  square  meter  (roughly  a  square  yard)  of  leaf  I 
ranges  according  to  circumstances  all  the  way  from  near  nothing  f 
up  almost  to  300  grams  (11  ounces),  the  generalized  average,  or  I 
conventional  constant,  is  50  grams  per  square  meter  (nearly  1 
2  ounces  per  square  yard)  per  hour,  i.  e.,  50  gm2h,  by  day  and  \  of 
this  quantity,  10  gm2h,  by  night,  which  equals  30  grams  per  square  1 
meter,  30  gm2h  (an  ounce  per  square  yard)  per  hour,  day  and    ^^^ 
night  together.    Upon  this  basis,  an  average  leaf  during  an  ordi-  . 
nary  summer  season  transpires  an  amount  of  water  equal  to  its  I/ 
own  area,  and  a  centimeter  (|  of  an  inch)  deep.    These  quantities 
are  well  worth  remembering. 

The  first  sensation  of  the  student  as  he  really  comprehends 
these  data,  especially  whenever  they  are  yielded  by  experiments 
of  his  own,  is  always  one  of  surprise  at  the  largeness  of  the  quan- 
tity. It  is,  indeed,  this  copiousness  of  transpiration,  rather  than 
the  existence  of  the  process,  which  is  the  remarkable  thing  about 
it;  and  it  helps  to  explain  a  number  of  more  or  less  familiar 
phenomena.  Thus,  the  rapidity  with  which  leaves  always  wilt 
when  cut  from  their  stems,  and  the  quickness  and  completeness 
with  which  plants  can  dry  out  the  soil  of  their  pots,  are  conse- 
quences of  transpiration.  In  this  way  some  plants  can  serve  as 
good  drainers  of  marshy  soils.  Thus  Eucalyptus  trees,  especially 
active  transpirers,  have  been  used  for  this  purpose  in  the  Roman 
Campagna  with  such  success  that  the  marshes  have  become 
freed  from  the  former  scourge  of  malaria-carrying  mosquitoes, 
and  therefore  habitable  by  man;  while  the  malaria-repelling 
virtue  often  ascribed  in  this  country  to  Sunflowers,  which  are 
sometimes  planted  around  dwellings  with  this  end  in  view,  has 
the  same  genuine  scientific  basis.  It  is  also  transpiration  condi- 
tions chiefly  that  determine  which  kinds  of  plants  can  be  grown 
in  dwellings  as  house  plants.  House  plants  are  by  no  means  the 


202  The  Living  Plant 

most  attractive  kinds  there  are,  but  are  the  most  attractive  that 
can  withstand  the  dryness  that  prevails  in  our  houses  in  winter, — 
a  dryness  that  is  due  not  so  much  to  the  heat  of  the  house  as  to 
the  fact  that  the  general  atmosphere  in  the  winter  has  a  very  low 
content  of  water  vapor.  A  house  plant  in  fact  is  one  whose 
transpiration  in  that  dry  heat  is  no  greater  than  can  be  com- 
pletely compensated  by  the  absorption  and  conduction  of  water 
from  the  soil.  And  this  relation  of  transpiration  to  conduction 
explains  another  notable  phenomenon  in  plant  nature,  namely 
the  limitation  in  the  height  of  trees,  which  in  general  are  just  so 
high  as  the  water  can  be  conducted  in  sufficient  abundance  to 
supply  the  transpiration  from  the  foliage.  When  that  height  is 
reached  the  tree  can  still  spread  out  laterally,  which  explains  the 
flat  tops  of  the  largest  Elms,  Maples,  Oaks  and  others,  and  of 
many  forest  trees  when  seen  from  mountain  tops.  A  transpira- 
tion effect  of  a  very  different  sort  is  displayed  by  a  good  many 
plants  in  the  early  spring.  It  is  a  fact  that  roots  absorb  water 
very  slowly  when  chilled,  and  if  they  are  kept  for  a  tune  at  a  low 
temperature,  while  leaves  and  stems  are  exposed  to  conditions 
favorable  for  transpiration,  as  is  effected  quite  easily  by  experi- 
ment, the  plants  will  wilt  very  rapidly.  These  very  conditions 
are  often  supplied  naturally  in  the  spring,  for  if  the  soil  remains 
frozen  or  very  cold  after  warm  bright  days  have  forced  out  the 
leaves,  or  if  a  cold  spell  that  chills  the  soil  is  followed  abruptly 
by  very  warm  bright  windy  days,  then  the  young  leaves  transpire 
so  much  faster  than  the  water  can  be  supplied  by  the  roots,  that 
they  become  dry-blasted  as  if  by  a  frost,  to  which  latter  cause, 
indeed,  this  effect  is  commonly  but  mistakenly  ascribed.  This  is 
the  explanation  also  of  the  fatal  browning  of  the  leaves  of  many 
ornamental  evergreens,  whose  leaves  are  awakened  to  active 
transpiration  before  the  roots  can  supply  the  water  they  need; 
and  it  is,  indeed,  a  chief  cause  of  winter-killing  generally.  And 
finally,  as  to  transpiration  effects,  there  is  one  more  way  in 
which  this  process  exerts  a  very  remarkable  influence  upon 


How  Substances  are  Transported  and  Removed      203 

plants;  for  the  necessity  that  it  be  regulated  and  minimized  in 
places  where  water  is  habitually  scanty,  as  occurs  conspicuously 
in  deserts,  has  resulted  in  the  development  of  protective  adapta- 
tions which,  as  the  weird  aspect  of  desert  plants  abundantly 
attests,  affect  the  forms,  sizes,  and  other  structural  features  of 
plants  more  profoundly  than  does  any  other  influence  whatsoever 


FIG.  69. — A  transpirograph  iii  action.  The  loss  of  a  gram  of  water  from  the  plant  permits 
that  end  of  the  balance  to  rise  and  close  an  electric  circuit ;  this  acts,  through  an  electro- 
magnet, to  force  a  pen  against  a  revolving  time-drum  (seen  on  the  left  of  the  stand), 
and  at  the  same  time  to  drop  a  spherical  gram  weight  from  a  cylindrical  reservoir 
into  the  box  under  the  scale  pan,  which  is  thus  depressed,  again  breaking  the  circuit. 
Thus  a  record  is  made  on  the  time-drum  at  each  moment  when  the  plant  has  lost  a 
gram  of  water. 

excepting  only  Photosynthesis.  But  this  subject  belongs  really 
with  a  later  chapter  (on  Protection),  where  it  will  be  treated  in 
detail. 

The  results  of  all  experiments  on  transpiration  show  remarkable 
variations  in  its  amount;  but  it  soon  becomes  evident  that  such 
variations  are  correlated  closely  with  changes  in  external  condi- 
tions. This  can  be  tested  by  weighing  the  plants  while  kept 
under  somewhat  extreme  conditions  of  heat  or  cold,  humidity  or 
dryness,  light  or  darkness;  and  the  results  are  all  the  clearer  if  one 
makes  use  of  some  form  of  self-recording  instrument,  one  of 


204  The  Living  Plant 

which,  called  a  Transpirograph  (a  little  thing  of  my  own,  by  the 
way)  is  shown  in  operation  in  the  accompanying  photograph 
(figure  69).  By  its  use  the  plant  is  made  to  write,  precisely  and 
continuously  for  days  together,  a  record  of  its  own  transpiration. 
Further,  there  also  exist  instruments,  invented  long  ago  for  use 
in  meteorological  stations,  which  write  continuous  records  of 
the  very  conditions  that  affect  transpiration,  viz.,  tempera- 
ture and  humidity,  while  light  is  recorded  by  a  special  method. 
When  the  cotemporaneous  graphs  of  transpiration  and  the 
external  conditions  are  plotted  together  upon  the  same  sheet,  as 
in  case  of  the  accompanying  graph  (figure  70),  the  relation  be- 
tween process  and  influencing  factors  is  displayed  in  a  way 
which  leaves  little  to  be  desired  in  the  direction  of  exact  and  ex- 
pressive exhibition  of  the  relation  between  this  physiological 
process  and  the  external  conditions.  Indeed,  I  am  accustomed 
to  use  this  study  with  my  own  students  as  an  example  of  a  well- 
nigh  ideal  piece  of  physiological  method,  whereby  Nature  is 
compelled  not  only  to  display,  but, even  to  write  down,  for  the 
edification  of  man,  the  tale  of  her  own  operations.  I  often  recall 
with  delight  the  remark  once  made  by  an  eminent  literateur  who 
happened  to  visit  my  laboratory  at  a  time  when  this  experiment 
was  in  progress.  As  soon  as  he  had  grasped  the  full  scope  of  the 
matter,  he  turned  away  with  this  comment, — "Well,  I  don't  see 
what  there  is  left  for  Nature  to  do  but  lay  down  and  holler."  In 
these  words  he  expressed  very  well  both  the  aim  and  the  joy  of 
scientific  investigation,  which  after  all  is  a  kind  of  great  game 
where  one  matches  wits  against  Nature,  and  generally  loses,  but 
now  and  then  wins  and  gathers  the  stakes,  which  consist  in  a 
share  of  her  jealously-kept  secrets. 

But  to  return  to  our  experiments  on  the  effects  of  external 
conditions  upon  transpiration,  they  show  these  results.  Heat 
increases,  and  cold  lessens  it.  Heat,  indeed,  may  hasten  tran- 
spiration to  such  a  degree  that  water  is  lost  from  the  leaves  much 
faster  than  the  roots  can  absorb  it  or  the  stems  conduct  it,  in 


206  The  Living  Plant 

which  case  a  wilting  results  even  though  water  is  plenty  in  the 
soil;  but  plants  thus  wilted  can  quickly  recover  when  the  weather 
grows  cooler,  for  then  the  absorption  and  conduction  catch  up, 
so  to  speak,  and  again  fill  the  leaf.  Light  increases,  and  darkness 
lessens  it.  This  harmonizes  with  our  transpiration  constants, 
which  showed  that  in  general  the  process  is  five  times  more  active 
in  daylight  than  at  night;  and  it  explains  why  plants  that  wilt  in 
the  day  recover  at  night.  Dryness  increases,  and  humidity  lessens 
it.  This  is  the  reason  why  most  kinds  of  plants  will  not  live  in  our 
houses,  the  air  of  which  is  so  dry  that  the  leaves  lose  their  water 
much  faster  than  roots  and  stems  can  supply  it,  no  matter  how 
plenty  in  the  soil.  It  explains,  too,  why  leaves  never  wilt  in  the 
weather  called  muggy,  no  matter  how  hot,  and  also  why  leaves 
that  are  wilted  recover  when  sprayed,  even  though  experiment 
proves  that  none  of  the  spray  is  absorbed.  As  to  other  external 
climatic  conditions,  their  influence  is  slight,  except  in  the  case  of 
the  wind,  which  always  promotes  it.  Thus  it  is  evident  that  in 
general  transpiration  is  promoted  by  the  very  same  factors  which 
favor  evaporation,  though  later  studies  have  shown  that  the 
parallel  does  not  hold  true  in  detail. 

We  must  now  consider  the  structural  basis  of  transpiration, 
with  which,  however,  the  reader  already  has  incidentally  made 
some  acquaintance.  If  he  will  recall  his  knowledge  of  the  cellular 
structure  of  the  leaf,  refreshing  his  memory,  perhaps,  by  another 
inspection  of  figure  2,  Plate  I,  B,  and  figure  54,  B,  it  will  be  clear 
that  every  cell  borders,  for  purposes  of  respiration  and  photosyn- 
thesis, upon  the  inter-cellular  air-system,  which  ramifies  through- 
out the  leaf  and  opens  to  the  outside  world  through  the  stomata, 
— the  little  slit-like  openings  through  the  otherwise  continuous 
epidermis.  Now  these  cells  are  all  gorged  with  water,  which 
saturates  their  walls;  and  where  these  border  on  the  air  spaces 
the  water  necessarily  evaporates.  The  vapor  thus  formed  satu- 
rates the  air  inside  of  the  leaf,  and  is  then  moved  by  the  force  of 
its  own  diffusion  along  the  passages  and  through  the  stomata  to 


How  Substances  are  Transported  and  Removed      207 

the  relatively  dry  atmosphere  outside.  Such  is  the  structural 
and  physical  basis  of  transpiration,  and  it  explains  perfectly  why 
heat,  which  is  an  evaporation  accelerator,  and  dryness  and  winds, 
which  are  diffusion  promoters,  increase  the  process. 

But  though  such  is  its  basis,  transpiration  is  really  not  so 
simple  as  this,  for  it  is  influenced  much  by  another  condition, 
and  that  is  the  number  and  size  of  the  stomata.  As  to  their  num- 
ber, that  varies  immensely  with  different  kinds  of  plants,  there 
being  none  at  all  on  the  upper  surface  of  a  good  many  leaves, 
while  on  lower  surfaces  they  vary  from  a  few  up  to  near  500  to 
every  square  millimeter  (one-twenty-fifth  of  an  inch),  with  a 
conventional  mean  at  100;  and  this  equals  no  less  than  100  mil- 
lions to  the  square  meter  (yard),  which  is  another  of  our  con- 
ventional constants.  And  it  is  worth  while  to  add  that  when  all 
of  the  stomata  are  open  their  widest,  about  one-hundredth  of  the 
whole  area  of  the  leaf  is  exposed.  As  to  the  size  of  the  stomata, 
that  not  only  varies  with  the  kinds,  but  in  each  kind  is  highly 
variable,  since  they  open  and  close,  from  near  a  circle  through  a 
narrowing  oval  to  a  slit  and  perhaps  no  passage  at  all,  by  the 
movements  of  two  bordering  cells  called  guard  cells.  These 
guard  cells,  as  shown  by  the  typical  example  pictured  herewith 
(figure  71),  are  of  aspect  distinctive  and  unmistakable,  with 
little  resemblance  to  others  of  the  epidermis.  They  are  usually 
somewhat  kidney-shaped,  forming  together  two  halves  of  an 
elongated  oval,  and  they  contain  chlorophyll.  Their  construction 
is  such,  as  figure  71,  lower,  illustrates,  that  the  natural  spring  of 
their  walls  tends  to  bring  them  together  and  close  up  the  stomatal 
slit ;  but  the  development  of  osmotic  turgescence  in  their  cavities 
rounds  them  out  so  that  they  separate,  thus  opening  the  slit. 
Now  this  turgescence  of  the  guard  cells  is  influenced  much  by  the 
quantity  of  water  contained  in  the  leaf,  rising  and  falling  there- 
with, so  that  when  water  is  plenty  the  stomata  tend  to  be  open, 
but  when  it  is  scarce  they  tend  to  be  closed.  Thus  it  seems  as  if 
the  guard  cells  ought  to  act  adaptively  as  regulators  of  transpira- 


208 


The  Living  Plant 


tion,  keeping  it  down  to  safe  limits  when  water  is  scanty,  but 
allowing  full  play  when  water  is  plenty.  The  turgescence  of  the 
guard  cells,  however,  is  influenced  also  in  another  way;  for 

they  (and  they  only  of  epidermal 
cells),  contain  chlorophyll,  which 
has  to  make  sugar  in  light  and 
thus  increase  their  turgescence 
and  cause  them  to  open  the  sto- 
mata.  This  arrangement  would 
explain  to  perfection  why  light 
increases  transpiration  so  greatly 
quite  apart  from  any  accompany- 
ing heat,  while  a  definite  ecologi- 
cal advantage  seems  equally 
clear,  viz.,  it  should  ensure  open- 
ing of  the  stomata  at  those  times 
when  the  demand  for  carbon  di- 
oxide is  the  greatest,  and  allow 
them  to  close  with  the  lessening 
of  this  need.  From  the  structure 
of  the  guard  cells,  therefore,  we 
should  expect  them  to  serve 
as  automatic  valves,  regulating 
FIG.  7i.—  Typical  guard  cells,  with  a  transpiration  adaptively  to  the 

stoma  between  them,  highly  magnified,  ov*orT,Q1      /.rmrlifirmc  •      anrl       fVmo 

in  surface  view  and  cross  section.      The  external      Conditions,      and 

lower  figure  shows  diagrammatically  in  {,heV  have  USUallv   been  regarded 

cross  section  the  method  by  which  the  J 

turgescent  rounding    of    their    cavities  by   botanists.       But    this    COnCCp- 

opens    the    stoma,  —  the    dotted    walls  .          ,                     ,                                    ,    , 

showing  the  closed,  and  the  unshaded  tlOn   nas  not    been    Sustained    by 

!«*«  studies,  which  have  shown 


L.  Kny,  and  the  lower  from  a  much-    so    much    irregularity,     and 
copied  diagram  by  Schwendener.) 

anomaly,  in  their  action  that  we 

have  to  remain  in  doubt  until  further  researches  shall  give  us 
the  truth.  Meantime  we  can  only  consider  that  any  regulatory 
action  they  may  have  is  clumsy  at  the  best. 


How  Substances  are  Transported  and  Removed      209 

Such  are  the  principal  facts  as  to  transpiration,  and  they  bring 
us  to  the  problem  of  its  physiological  meaning,  upon  which  also 
there  is  uncertainty.  The  older  explanation  argued  thus: — 
plants  need  in  all  parts,  and  especially  their  leaves,  certain 
minerals  from  the  soil:  their  only  possible  method,  apparently, 
of  raising  these  minerals  to  the  places  of  use  consists  in  absorbing 
and  transferring  them  in  water,  and  evaporating  the  latter  to 
leave  them  behind :  some  of  the  minerals  are  so  scarce  that  plants 
hardly  ever  can  get  as  much  as  they  need:  the  more  copious  the 
transpiration  the  more  minerals  are  raised;  presumably,  there- 
fore, transpiration  is  the  mineral-raising  process  and  is  the  more 
efficient  the  more  copious  it  is.  On  this  assumption,  plants  would 
be  expected  to  develop  adaptations  for  promoting  transpiration, 
and  a  great  many  such  have  actually  been  claimed  to  exist,  as 
will  presently  appear.  A  second  explanation  argues  thus: — the 
stomata  exist  primarily  for  admission  of  carbon  dioxide  needed 
in  photosynthesis  (they  occur,  in  general,  only  in  green  tissues) : 
when  open  for  this  purpose,  evaporation  and  diffusion  of  water 
will  necessarily  take  place  from  the  saturated  cell-walls  of  the 
interior  of  the  leaf  as  a  purely  physical  operation  which  the  plant 
has  no  power  to  prevent:  presumably,  therefore,  transpiration  is 
merely  an  incidental  physical  accompaniment  of  photosynthesis, 
a  kind  of  necessary  evil,  as  it  were.  Upon  this  explanation 
adaptations  would  be  expected  for  its  prevention,  especially  of  a 
kind  which  would  not  interfere  with  photosynthesis;  and  of  these 
a  good  many  have  been  described,  as  we  shall  note  in  the  follow- 
ing chapter.  This  explanation  accounts  best  for  most  of  the 
phenomena,  and  is  the  one  that  is  generally  accepted  at  present. 
A  third  explanation  argues  thus: — when  full  sunlight  falls  on  a 
leaf,  it  beats  thereon  with  an  energy  overwhelmingly  greater 
than  the  leaf  can  employ  in  its  work  (for  it  actually  uses  no  more 
than  some  three  per  cent):  this  energy,  both  light  and  heat, 
would  work  disaster  to  the  living  protoplasm  unless  dissipated 
in  some  manner:  evaporation  is  a  highly  effective  method  of 


210 


The  Living  Plant 


energy-dissipation:  presumably,  therefore,  transpiration  is  an 
adaption  to  protection  against  injury  from  the  over-plentiful 
energy  of  sunlight.  Each  of  these  explanations  has  its  merits  and 
its  difficulties,  and  no  one  alone  is  sufficient.  Probably  the  truth 
will  be  found  to  involve  some  participation  of  all  three;  transpira- 
tion may  be  fundamentally  a  process  which  the  plant  cannot 
prevent,  but  that  is  no  reason  why  the  plant  cannot  employ 
it,  and  even  develop  it  highly,  as  an  easy  method  of  raising  its 
requisite  minerals,  and  a  convenient  means  for  the  dissipation  of 
superfluous  energy.  But  this  question,  too,  is  one  of  the  many 
whose  solution  lies  with  the  future. 

Transpiration,  however,  is  not  the  sole  method  by  which  water 
is  removed  from  the  plant.  Everybody  has  noticed  the  clear 
shining  drops  which  bejewel  the  margins  of  Grape  leaves  on 
mornings  that  follow  hot  days  and  cool  nights;  these  drops  are 
commonly  thought  to  be  dew  but  are  not.  They  show  very 
strikingly  also  on  young  plants  of  Nasturtium  and  seedlings  of 
Grasses,  where  they  can  be  made  to  appear  whenever  desired, 
simply  by  covering  the  actively-transpiring  plants  for  a  few 
minutes  by  a  cooled,  darkened,  or  dampened  bell-jar.  In  a  great 
many  other  plants,  too,  the  drops  appear  and  are  mistaken  for 
dew.  The  slender  wet  streaks  often  seen  on  the  leaves  of  the 
Cannas  just  after  sundown,  come  from  similar  marginal  drops; 
and  a  tropical  plant  is  said  to  exist  from  which  water  is  projected 
in  a  very  fine  jet.  In  all  of  these  cases  the  water  is  known  to 
come  from  inside  the  plant,  and  the  process,  known  physiologic- 
ally as  guttation,  is  a  result  of  the  following  conditions.  On  very 
warm  days  the  vigorous  transpiration  is  accompanied  by  an 
equally  energetic  absorption  and  transfer,  but  the  comparatively 
sudden  check  to  transpiration  caused  by  the  cool  of  the  evening 
does  not  at  once  affect  the  absorption;  therefore  water  continues 
to  be  forced  into  the  stems  and  leaves  to  an  extent  which  might 
prove  a  serious  detriment  were  it  not  for  an  avenue  of  escape  pro- 
vided by  openings  existing  in  the  ends  of  the  veins,  for  it  is  here 


How  Substances  are  Transported  and  Removed      211 

that  the  water-drops  always  appear.  Guttation,  therefore,  is  a 
kind  of  a  safety-device  for  the  plant  even  if  transpiration  is  not. 
Furthermore,  it  happens  at  times  that  roots  keep  their  vitality 
long  after  the  stems  have  died,  and  continue  to  force  up  water 
which  can  find  an  outlet  only  through  rifts  that  it  makes  in  the 
withering  stems.  Besides,  in  cold  weather  all  stems  tend  of 
course  to  contract,  thus  squeezing  from  such  rifts  any  over- 
abundant water  they  may  happen  to  contain.  When  water  from 
either  of  these  sources  is  forced  out  in  cold  weather,  it  freezes  in 
lines,  which  soon  become  flat  plates  as  more  and  more  issues 
from  the  stem,  pushing  the  already  formed  ice  before  it ;  and  this 
is  the  origin  of  the  ice  crystals  or  shells,  often  of  great  beauty  and 
commonly  mistaken  for  "  frost,"  which  are  seen  on  the  stems  of 
some  plants  in  the  early  part  of  the  winter.* 

If  I  seem  to  have  dwelt  over-long  on  this  matter  of  water- 
removal  from  the  plant,  I  claim  in  explanation  that  the  process, 
because  of  the  profundity  of  its  effect  upon  plant-structure  and 
habit,  is  worth  all  the  space  I  have  taken;  and  the  later  chapter 
on  Protection  will  help  to  support  this  conclusion.  But  now  we 
are  ready  to  proceed  to  the  topics  remaining,  of  which  the  re- 
moval or  excretion  of  substances  other  than  water  comes  naturally 
next.  These  excretions  belong  to  four  different  classes.  First,  of 
course,  are  the  gases,  for  oxygen  is  an  excretion  in  photosynthesis, 
and  carbon  dioxide  in  respiration.  But  the  subject  is  simple,  for 
they  pass  off  by  diffusion,  either  through  stomata  and  lenticels 
of  leaves  and  stems,  or  in  solution  through  the  wet  epidermis  of 

*  A  conspicuous  case  occurs  in  Helianthemum  canadense,  commonly  called  Frost- 
weed,  which  is  described  in  Gray's  Manual  of  Botany  thus:  "Late  in  autumn  crystals 
of  ice  shoot  from  the  cracked  bark  at  the  base  of  this  and  the  next  species,  whence 
the  popular  name."  Another,  and  even  more  striking,  example  is  the  Dittany 
(Cunila  Mariana,  or  origanoides) ,  in  which  the  ice-forming  habit  has  thus  been  de- 
scribed: "Our  Cunila  has  attached  to  the  stem  a  shell-work  of  ice,  of  a  pearly  white- 
ness, beautifully  striated,  sometimes,  like  a  series  of  shells  one  in  another — at  others 
curved  round  on  either  side  of  them  like  an  open,  polished,  bivalve;  then,  in  others, 
again,  curled  over  in  every  variety  of  form,  like  the  petals  of  a  tulip."  (J.  Stauffer, 
quoted  in  the  Botanical  Gazette,  XIX,  1894,  326.) 


212  The  Living  Plant 

the  roots.  Second,  are  various  minerals,  which  in  part  are  useless 
materials  absorbed  along  with  the  useful  kinds,  and  in  part  are 
by-products  of  chemical  changes  inside  of  the  plant.  For  their 
removal  plants  have  no  regular  excretory  system  as  animals 
have,  though  a  partial  substitute  exists  in  the  fall  of  the  leaves 
and  the  bark,  which  thus  remove  crystalline  matters  they  con- 
tain. Other  minerals  are  left  behind  as  crystals  in  the  old  dead 
cells  when  the  living  protoplasm  advances  into  the  new  ones  it 
forever  is  building  (compare  figure  41).  Third,  are  the  root- 
poisons,  little  known  to  us  yet  and  even  by  some  experts  not  be- 
lieved to  exist.  They  appear  to  be  highly  complex  organic  sub- 
stances, slow  of  diffusion  and  drainage,  and  poisonous  to  the  roots 
which  produce  them  though  not  necessarily  to  different  kinds; 
and  this  fact  gives  a  new  explanation  of  the  advantage  of  rotation 
of  crops  and  of  letting  a  soil  lie  fallow.  Fourth,  is  extra-floral 
nectar,  apparently  identical  in  composition  and  mode  of  forma- 
tion with  the  nectar  of  flowers,  which  performs  the  invaluable 
service  of  attracting  cross-pollinating  insects,  as  later  we  shall 
note  in  detail.  The  extra-floral  nectaries  are  very  tiny  structures, 
sometimes  marked  by  blotches  of  color,  occurring  commonly  at,  or 
near,  the  bases  of  leaves  in  young  plants  (e.  g.  in  some  Ferns, 
Horse  Beans,  Castor  Beans  and  others),  or  with  the  spines  (in 
Cactus),  and  elsewhere.  They  have  been  supposed  to  attract 
small  ants  which  may  perform  some  ecological  service;  but  the 
evidence  thereon  is  so  unsatisfactory  that  it  seems  best  to  place 
this  nectar  for  the  present  among  the  excretions,  though  surely 
it  is  a  puzzling  sort. 

So,  and  by  such  means,  are  substances  removed  from  plants. 
The  reader  knows  also  in  what  ways  they  are  absorbed.  Between 
absorption  and  removal  they  have  to  be  transported,  often  for 
very  long  distances;  and  this  is  the  matter  which  next  needs 
attention. 

The  principal  substance  to  be  transported  is  water,  of  which 
transpiration  demands  so  great  a  supply  that  it  has  to  be  moved 


How  Substances  are  Transported  and  Removed      213 

in  a  copious  and  continuous  current  through  the  plant.  This 
involves  of  course  a  highly  efficient  water-carrying  mechanism, 
which  we  should  first  consider.  The  principal  feature  thereof  is 
the  ducts,  which  are  tubes,  beginning  near  the  tips  of  the  roots 
(figure  53)  and  running  in  bundles  throughout  the  length  of  the 
stem  to  the  leaves,  as  our  earlier  generalization  of  the  system 
so  clearly  illustrates  (figure  54,  A) ;  and  here  they  end  in  little  areas 
of  green  tissue,  as  we  have  noted  already  in  the  description  of  the 
leaf.  Structurally,  the  individual  ducts  are  short,  but  the  end  of  / 
each  one  lies  against  the  end  of  another  with  only  a  thin  partition  / 
between;  and  therefore  the  practical  effect  is  that  of  a  continuous  I 
tube  with  occasional  thin  cross  partitions.  When  roots  and  stems  ^ 
are  young  and  flexible,  the  soft  walls  of  the  ducts  are  supported 
inside  by  ringed  or  spiral  thickenings,  which  keep  the  cavities 
open  when  the  young  roots  or  stems  become  sharply  bent  back 
by  accident,  and  also  against  the  turgescent  pressure  of  neighbor-, 
ing  cells.  The  ducts  formed  later,  however,  when  the  tissues  are 
thicker  and  harder,  have  not  the  spirals,  but  stiff  bands  or  a  fret 
work,  or  even  a  uniform  thickening,  pierced  by  thin  areas  for 
the  escape  of  some  water  to  the  neighboring  tissues.  These  dis- 
tinctive features  of  ducts  are  very  well  shown  in  the  picture 
given  herewith  (figure  72;  also  54,  C). 

We  turn  now  to  the  study  of  the  transfer  of  water  through  the 
plant,  or,  as  it  may  also  be  expressed,  the  forces  impelling  the 
ascent  of  sap.  Transpiration  makes  very  great  demands  for  a 
water  supply,  especially  in  lofty  and  broad-leaved  trees,  and  in 
weather  that  is  bright,  dry,  and  windy.  By  what  forces  is  so 
weighty  a  volume  of  water  raised  so  quickly  to  a  height  so  great? 
Recently  I  had  occasion  to  calculate  the  work  done  in  a  day  in 
transferring  the  water  from  roots  to  leaves  in  one  of  the  largest 
kind  of  trees,  and  I  found  it  was  just  about  equal  to  that  which 
would  be  done  by  a  man  in  carrying  500  large  pailfuls  of  water 
up  a  ten-foot  flight  of  stairs  within  ten  hours.  This  is  nearly  a 
pailful  a  minute  for  ten  hours  without  cessation,  my  figures  being 


214 


The  Living  Plant 


expressed  in  this  form  in  order  to  bring  the  matter  home  to  my 
students.  Now,  strangely  enough,  the  botanists  are  not  yet 
agreed  either  as  to  the  source  of  the  energy  or  the  precise  physical 
method  by  which  this  considerable  work  is  accomplished;  and  in 
default  of  precise  information  I  can  only  present  to  the  reader  a 
synopsis  of  such  data  as  we  possess,  along  with  some  comments 
on  their  probable  bearing.  And  here  follow  the  principal  explana- 
tions which  have  been  offered  for  the  physics  of  sap  ascent. 


FIG.  72. — A  generalized  drawing  of  the  tissues  of  a  typical  stem,  showing  the  water- 
carrying  ducts  (the  three  larger  tubes),  and  a  food-carrying  sieve-tube  (the  single 
dot-lined  tube) ,  with  the  associated  tissues.  (Copied  from  Kerner's  Pflanzenleben.) 

1.  Root  pressure. — In  the  preceding  chapter  it  was  shown  that 
roots  absorb  water  osmotically  and  forcibly  start  it  up  the  ducts. 
But  this  pressure,  which,  in  some  greenhouse  plants  has  been 
found  sufficient  to  raise  water  40  to  50  feet,  and  in  trees  up  to 
80  feet,  is  wholly  insufficient  to  explain  the  ascent  when  trees 
reach  400  feet,  as  they  do  in  some  kinds  of  Australian  Eucalyptus ; 
and  therefore  this  cannot  be  the  explanation. 


How  Substances  are  Transported  and  Removed      215 

2.  Atmospheric  pressure. — This  will  suffice,  when  the  suitable 
conditions  are  provided,  as  they  are  in  a  pump,  to  raise  water 
some  32  feet,  but  no  more;  in  the  plant,  however,  the  requisite 
conditions  are  wanting,  while  this  height  is  obviously  quite  in- 
adequate.   Therefore  this  cannot  be  the  explanation. 

3.  Capillarity. — This  is  the  power,  as  the  reader  will  recall, 
by  which  water,  driven  by  its  own  internal  molecular  energy, 
rises  in  small  tubes,  the  higher  the  smaller  the  tube.    But  even 
the  slenderest  ducts  known  to  occur  in  plants  are  not  small 
enough  to  raise  the  water  more  than  a  few  feet  even  if  all  the  other 
conditions  were  most  favorable,   which  indeed  they  are  not. 
Therefore  this  cannot  be  the  explanation. 

4.  Imbibition. — This   was   the   favorite    theory   of   the   great 
botanist  Sachs,  who  defended  it  to  the  end  of  his  life.    He  con- 
ceived of  the  wall-system  of  the  plant  as  a  kind  of  gigantic  con- 
tinuous membrane,  extending  all  the  way  from  the  root  hairs  to 
the  cells  of  the  leaf;  into  this  membrane,  by  forces  and  method 
already  considered,  water  was  absorbed  by  imbibition,  and  raised 
by  the  same  energy,  to  be  finally  removed  by  evaporation  at 
the  leaf-cells.    The  theory  is  simple  and  plausible,  but  is  shattered 
by  one  fatal  fact, — viz.  it  requires  that  the  transpiration  stream 
shall  move  in  the  walls  of  the  ducts,  not  their  cavities  (which 
Sachs  took  simply  for  reservoirs),  whereas  experiment  proves 
beyond  question  that  the  water  does  move  in  the  cavities.    There- 
fore this  cannot  be  the  explanation. 

5.  Propulsion. — This    theory    maintains    that    the    water    is 
forced  or  propelled  upwards  by  some  action  of  the  living  cells 
distributed  along  the  course  of  the  ducts,  each  living  cell  being 
supposed  to  draw  water  from  a  lower  duct  and  force  it  out  into  a 
higher.     It  really  is  an  extension  of  root  pressure  to  the  whole 
stem,  the  living  cells  passing  water  from  one  duct  to  another 
precisely  as  the  root  hairs  and  cortex  pass  it  from  the  soil  into  the 
ducts, — and  by  the  very  same  physical  power  and  method,  which 
is  still  unknown  in  detail.    It  differs  from  the  preceding  explana- 


2i6  The  Living  Plant 

tions  in  this,  that  it  involves  the  activity  of  cells  which  are  alive ; 
and  herein  also  it  meets  its  greatest  difficulty,  because,  accord- 
ing to  some  experimenters,  when  the  living  cells  are  killed  by 
suitable  methods,  the  water  continues  to  ascend,  at  least  for 
some  time.  Therefore,  they  say,  this  cannot  be  the  explanation. 
But  others  are  not  convinced  that  the  cells  are  really  all  killed  in 
these  experiments,  and  hold  that  this  explanation  is  substantially 
correct. 

6.  Traction. — This,  the  most  recent  explanation,  has  been 
worked  out  by  a  botanist,  Dixon,  and  a  physicist,  Joly,  working 
in  collaboration,  and  is  often  known  by  their  name.  It  maintains, 
in  brief,  that  water  in  very  thin  threads  holds  together,  by  the 
force  of  its  own  internal  cohesion,  with  a  tenacity  sufficient  to  make 
it  as  strong  as  a  solid  fiber  or  wire;  wherefore  the  thin  threads 
of  water  in  the  ducts  can  actually  sustain  their  own  weight  for 
a  length  as  great  as  the  height  of  the  tallest  trees.  These  threads 
being  practically  continuous  from  the  tips  of  the  roots  to  the 
cells  of  the  leaves,  hang,  as  it  were,  from  the  leaf-cells,  into  which 
they  can  be  lifted  by  any  power  that  can  remove  the  water  from 
those  cells.  This  power  is  supplied  by  the  energy  of  evaporation 
in  transpiration,  which  latter  process,  therefore,  lifts  or  drags 
the  water  threads  up  the  ducts  much  as  a  man  on  a  roof  would 
pull  up  a  rope  from  the  ground.  On  this  view  the  energy  which 
raises  the  water  in  the  tree  is  the  same  which  lifts  it  to  the  clouds. 
This  theory  finds  its  chief  difficulty  in  the  lack  of  complete  demon- 
stration that  the  water  can  thus  cling  together  in  threads  of  such 
great  length,  and  it  has  not  been  universally  accepted. 

It  sometimes  appears  as  if  the  extent  of  our  knowledge  of  any 
subject  were  inversely  proportional  to  its  importance.  At  all 
events  we  found  this  to  be  true  of  the  structure  of  protoplasm, 
and  it  also  seems  true  of  this  subject  of  sap  ascent.  And  at 
present  there  is  a  pause  in  the  advance  of  our  knowledge  thereof. 
With  this  subject,  as  with  others,  we  find  out  everything  that 
existent  methods  of  investigation  can  yield,  then  turn  for  a  tune 


How  Substances  are  Transported  and  Removed      217 

to  other  matters.  Presently,  however,  somebody,  working  per- 
haps in  a  quite  different  field,  chances  upon  some  new  method 
that  happens  to  be  applicable  to  this  subject,  to  which  students 
then  turn  once  more,  and  make  another  long  step  in  advance. 
The  very  fact  that  all  knowledge  thus  grows  by  appreciable 
stages  makes  it  all  the  more  interesting  to  follow;  and  the  watch- 
ing for  such  new  knowledge,  and  the  grasping  it  when  it  appears, 
constitute  the  principal  charm  of  the  scientific  life. 

There  remains  but  one  other  point  in  connection  with  the 
transfer  of  water.  The  current  must  supply  not  only  the  tran- 
spiration loss,  but  all  the  working  needs, — chemical,  osmotic  and 
other, — of  the  various  tissues  besides.  This  matter,  however,  is 
simple,  for  all  kinds  of  ducts  possess  plenty  of  thin  places  through 
which  the  water  can  pass  outward,  after  which,  by  imbibition 
and  osmosis,  it  gradually  penetrates  from  cell  to  cell  throughout 
all  of  the  tissues  that  need  it.  And  with  the  water  in  this  way  go 
the  various  minerals  in  solution,  which  explains  their  transporta- 
tion, as  well  as  their  absorption,  by  the  plant. 

From  the  transport  of  water  and  minerals  we  turn  to  that  of 
the  various  food-substances  made  in  the  plant, — a  subject  known 
in  plant  physiology  as  translocation.  The  subject  is  comparatively 
simple.  In  the  first  place  such  substances  travel  invariably  in 
solution;  and  substances  which  are  not  soluble  in  water  never 
move  from  their  places  of  formation.  The  very  physical  nature 
of  some  substances,  e.  g.  the  sugars,  makes  them  naturally  soluble, 
but  others,  viz.  starches,  oils,  cellulose,  and  most  proteins,  are  for 
the  same  reason  insoluble.  In  such  cases  solubility  is  obtained, 
for  purposes  of  translocation,  by  their  conversion  (or  hydrolysis) 
into  closely-related  substances  which  are  soluble, — thus  starch 
and  cellulose  into  sugar,  oils  into  fatty  acids,  insoluble  proteins 
into  peptones.  These  changes  are  effected  by  those  remarkable 
substances  called  enzymes,  whose  method  of  action  we  have  con- 
sidered in  the  chapter  on  Metabolism.  The  enzymes  are  widely 
scattered  through  plants,  and  some  of  them  are  identical  with  the 


218  The  Living  Plant 

digestive  juices  (diastase,  pepsin)  found  in  the  alimentary  system 
of  animals;  for  the  solution  or  hydrolysis  of  insoluble  foods  by 
enzymes  constitutes  digestion  in  plants  just  as  truly  as  in  animals. 
This  digested  material  is  then  in  suitable  condition  for  transporta- 
tion, which  takes  place  in  two  ways.  First,  it  may  be  carried 
with  an  onward-moving  water  current,  as  happens  with  the  sap 
in  the  spring  (witness  the  Sugar  Maple),  when  the  food  stored 
for  the  winter  in  the  roots  or  lower  trunk  of  the  tree  diffuses  from 
the  storage  cells  into  the  sap  current  and  rises  therewith.  Sec- 
ond, it  may  travel  by  diffusion  alone,  for  a  substance  dissolved 
in  water  is  in  perfect  physical  condition  for  diffusion, — that 
is,  has  the  power  and  the  tendency  to  move  outward  and  on- 
ward, by  its  own  diffusive  energy,  from  places  of  greater  to 
places  of  lesser  concentration  until  equilibrium  is  established. 
When,  furthermore,  the  substance  is  being  produced  at  one  place, 
as  occurs  with  sugar  in  the  leaves  during  photosynthesis,  and  is 
being  removed  in  another,  as  occurs  in  places  of  storage  where  it 
is  converted  into  insoluble  starch,  then  a  steady  diffusion  current 
is  established  between  the  place  of  production  and  the  place  of 
use.  And  it  is  by  such  diffusion  currents  that  most  of  the  trans- 
location  of  food-substances  through  the  plant  is  effected,  though 
it  is  to  be  remembered  that  diffusion  alone,  from  its  very  nature, 
can  never  completely  empty  a  part.  This  explains  why  some 
sugar  and  other  food  materials  remain  in  autumn  leaves  when 
they  fall. 

This  translocatory  diffusion  proceeds  in  part  from  cell  to  cell 
through  the  walls,  the  protoplasmic  linings  thereof  being  adjusted 
(by  appropriate  chemical  modification  or  intermicellar  spacing, 
as  noted  earlier  under  Absorption)  to  permit  the  passage  of  the 
molecules  of  the  substance;  and,  given  time  enough,  there  is  no 
limit  to  the  distance  that  substances  may  thus  pass  in  solution. 
Obviously,  however,  such  translocation  through  long  distances 
must  be  greatly  facilitated  if  long  tubes  replace  the  short  cells; 
and  such  a  system  is  actually  found  in  the  elongated  sieve-tubes 


How  Substances  are  Transported  and  Removed      219 

which  are  very  well  illustrated  in  our  figure  72.  These  sieve-tubes 
accompany  the  ducts  all  through  the  plant  from  root-tips  to 
stem-tips  and  leaf-cells,  as  our  generalized  plant  illustrates  so 
clearly  (figure  54),  thus  forming  a  part  of  the  same  fibro-vascular 
bundles.  But  sieve-tubes  are  more  slender  than  ducts,  and  unlike 
them  have  thin  soft  walls,  and  a  continuous  lining  of  protoplasm; 
while  the  occasional  cross  partitions,  thicker  than  the  walls,  are 
perforated  by  openings  in  a  way  which  has  given  these  structures 
their  name  (figure  54,  C).  The  presence  of  this  protoplasmic 
lining  in  the  sieve-tubes  when  diffusion  alone  does  not  require  its 
presence  at  all,  suggests  that  it  plays  some  part  in  helping  to  force 
substances  along  the  tubes,  perhaps  in  a  manner  analogous  to  the 
way  in  which  the  food  is  moved  along  the  intestines  of  animals; 
but  no  such  action  has  been  proven.  Doubtless  the  movement  is 
aided  materially  by  the  swaying  of  branches  in  the  wind,  and, 
when  it  is  downwards,  by  gravitation;  but  these  influences  are 
obviously  both  incidental  and  irregular,  and  diffusion  is  the  only 
motive  force  in  translocation  that  we  surely  know.  The  reader, 
therefore,  must  visualize  this  process  as  one  of  constant  diffusion 
along  the  sieve-tubes.  It  is  not  an  onward  movement  of  the 
solution  they  contain,  but  a  movement  of  the  sugar  and  other  dis- 
solved substances  through  water  that  is  standing  still,  a  process 
in  great  contrast  with  the  onward  rush  of  sugar-carrying  sap  in 
the  spring.  The  method  of  this  diffusion,  by  the  way,  is  illus- 
trated diagrammatically  in  figure  6. 

The  sieve-tubes,  in  which  translocation  of  food  principally 
proceeds,  lie  in  the  inner  bark  of  woody  plants,  down  through 
which,  accordingly,  all  summer  long,  there  is  a  constant  move- 
ment of  food-substances  towards  the  roots  or  other  underground 
parts  devoted  to  winter  storage.  That  this  is  really  the  path  is 
easily  proven  by  experiment,  such  for  instance  as  removing  a 
narrow  ring  of  the  bark,  or  constricting  it  by  a  metal  ring.  This 
often  happens  by  accident  in  Botanical  Gardens  where  the  en- 
circling wires  which  support  the  labels  are  left  too  tight.  In  all 


22o  The  Living  Plant 

such  cases  the  obstruction  in  the  bark  causes  an  accumulation 
of  the  food  just  above,  with  a  resultant  swelling  of  the  tissues 
that  often  is  very  prominent.  The  same  thing  happens  also 
naturally  where  a  twining  stem,  such  as  that  of  a  Bittersweet, 
tightly  constricts  a  growing  tree,  in  which  cases  the  swelling  stem 
always  shows  a  very  much  greater  enlargement  above  than  below 
the  vine. 

Such  is  the  method  whereby  food  materials  are  transported 
from  their  places  of  formation  to  the  places  of  storage  end  use. 
The  same  general  method  explains  the  transport  and  accumula- 
tion of  all  those  special  substances,  usually  of  definite  and  adapt- 
ive functions,  which  we  call  secretions, — the  volatile  oils,  nectar, 
some  coloring  matters,  and  others  which  have  been  considered 
in  the  chapter  on  Metabolism. 

This  is  really  the  place  to  bring  this  particular  chapter  to  a 
natural  conclusion;  and  it  is  truly  a  pity  that  it  cannot  be  done. 
For  somewhere  in  the  book  we  have  to  consider  the  prominent 
subject  of  the  cellular  anatomy  of  stems,  and  this  is  the  most 
suitable  place.  However,  the  matter  is  not  indispensable  to  a 
clear  understanding  of  the  chapters  that  follow,  and  therefore 
the  reader  may  skip  the  remainder  of  this  chapter  if  he  wishes. 
And  if  the  said  reader  should  ask  why  I  do  not  skip  it  myself,  I 
would  answer  that  the  integrity  of  my  subject  requires  its  pres- 
ence. For  with  regard  to  this  book  I  feel  with  Nehemiah  Grew, 
who  wrote  more  than  two  centuries  ago  in  the  dedication  to  his 
great  work  on  the  Anatomy  of  Plants, —  "Not  I,  but  Nature 
speaketh  these  things." 

If,  accordingly,  in  pursuit  of  a  knowledge  of  the  anatomy  of 
stems,  one  cuts  with  a  sharp  knife  a  clean  section  across  any 
young  stem,  he  can  always  discover  the  ends  of  the  fiber-like  veins 
distributed  in  a  uniform  ground-work  of  tissue.  And  if,  further- 
more, he  makes  a  thin  section  from  a  typical  young  stem,  such  as 
Castor  Bean,  and  magnifies  it  moderately,  he  will  have  before 
him  such  an  appearance  as  is  pictured  herewith  (figure  73). 


How  Substances  are  Transported  and  Removed      221 

while  a  typical  stem  is  shown  generalized  in  our  later  figure  139  B. 
Among  the  many  cellular  elements  in  the  symmetrical,  almost 
geometrical  structure  thus  displayed,  it  is  easy  to  identify  the 
bundles  of  ducts  from  their  relatively  large  size  and  their  obvious 
resemblance  to  the  cut  ends  of 
round  tubes.  Associated  with 
the  ducts,  and  a  little  way  re- 
moved towards  the  outside  of 
the  stem,  lie  clusters  of  smaller, 
thinner-walled,  and  more  angu- 
lar cells,  which  are  also  the  cut 
ends  of  long  tubes,  the  food- 
carrying  sieve-tubes;  while  be- 
tween sieve-tubes  and  ducts  lie 
two  or  three  layers  of  small 
squarish  cells  presenting  an 
aspect  which  later  the  reader  FlG  73._Cross  section  of  a  young  stem  of 

Will     learn     to     associate     With        ^  Cast°'  Bean'  magnified  about  twenty 

times.     (Copied,  reduced,  from  a  drawing 

growth,  for  they  are  the  Cam-  by  H.  O.  Hanson,  in  Curtis'  Nature  and 
i  .  ..  .  .  ,  „  Development  of  Plants.) 

bium  cells  which  form  new  ducts 

and  sieve-tubes  as  long  as  the  plant  lives.  Ducts,  sieve-tubes  and 
cambium,  to  which  often  are  added  strengthening  fibers,  grow 
all  or  a  part  of  them  together  in  bundles,  forming  fibro-vascular 
bundles  which  are  identical  with  the  veins, — both  the  kind  that 
can  be  seen  in  young  translucent  stems,  and  also  those  familiar 
in  leaves.  The  bundles  begin,  as  our  generalized  picture  of  the 
conducting  system  illustrates  (figure  54),  near  the  ends  of  the 
roots,  where  they  consist  of  a  few  ducts  and  sieve-tubes  only; 
farther  back  they  acquire  cambium  and  fibers  and  enlarge  greatly 
in  size;  in  the  stem  they  branch  at  the  nodes  and  run  out  to  the 
leaves,  when  they  fringe  away  gradually  to  the  veinlets,  each  of 
which  ends  as  a  single  duct  and  sieve-tube  in  the  midst  of  one  of 
the  ultimate  areas  of  green  tissue. 

The  fibro-vascular  bundles  have  not  only  this  definite  com- 


222  The  Living  Plant 

position,  but  a  definite  arrangement  in  the  stem,  where  they 
lie  in  a  ring,  as  our  pictures  illustrate  (figures  73,  139  B).  The 
tissue  in  which  they  are  embedded  consists  mostly  of  thin-walled 
cells,  of  rounded  or  polyhedral  shapes.  The  part  thereof  lying 
inside  of  the  ring  of  bundles  makes  up  the  pith,  which  is  com- 
monly utilized  for  storage;  that  between  the  bundles  constitutes 
the  beginnings  of  structures  later  to  be  considered  as  the  medul- 
lary rays;  while  the  tissue  outside  of  the  bundles  forms  the  cortex, 
which  contains  some  chlorophyll,  and  aids  in  the  photosynthetic 
work.  This  cortex,  by  the  way,  is  continuous  and  morphologically 
identical  with  the  green  tissue  of  the  leaf;  and  one  can  form  a  very 
useful  and  reasonably  accurate  conception  of  the  anatomical 
relations  of  stem  and  leaf  by  imagining  that  one  of  the  fibro- 
vascular  bundles  of  the  stem  is  snipped  out  from  among  its 
neighbors,  and,  with  its  adherent  cortex,  bent  outward  at  right 
angles  to  the  stem  and  then  flattened  and  fringed  out  to  a  network 
which  the  green  tissue  surrounds  and  fills  in.  But  as  to  our 
stem,  outside  of  the  tissues  aforementioned  comes  the  single  layer 
of  epidermis,  physiologically  the  plant's  skin,  with  its  distinctive 
flat  chlorophylless  cells  pierced  here  and  there  by  the  stomata. 
Finally,  sometimes  in  connection  with  the  sieve-tubes,  some- 
times as  a  ring  or  as  scattered  islands  in  the  cortex,  or  just  under 
the  epidermis,  occur  masses  of  very  thick-walled  cells,  showing 
long  and  pointed  when  seen  lengthwise,  which  are  the  important 
fibers  that  give  strength  to  the  stem.  Howsoever  these  fibers  are 
distributed,  there  is  always  one  constant  feature  about  their  posi- 
tions, that  they  tend  to  keep  close  towards  the  outside  of  the 
stem.  And  the  reason  therefor  is  sufficiently  plain, — it  is  a 
fundamental  principle  of  mechanics  that  any  given  amount  of 
strengthening  material  exerts  its  greatest  supporting  effect  against 
lateral  strains  if  disposed  in  the  form  of  a  hollow  cylinder  or  tube, 
which  is  the  reason  why  columns  used  in  building  construction 
are  hollow,  not  solid,  why  a  bicycle  frame  is  constructed  of  tubes, 
not  of  rods,  and  why  a  great  tree  can  stand  as  a  mere  shell  of 


How  Substances  are  Transported  and  Removed      223 

wood  long  after  its  center  has  rotted  away.  It  is  true,  this  prin- 
ciple would  require  for  greatest  efficiency,  that  the  fibers  should 
lie  on  the  very  outside,  as  indeed  they  do  in  some  cases;  but  such 
an  arrangement  would  prevent  all  access  of  light  and  therefore 
the  use  of  the  surface  for  spreading  of  chlorophyll.  It  is  easy  to 
understand  how  the  plant  could  find  it  advantageous  to  sacrifice 
a  trifle  of  effectiveness  in  the  strengthening  system  for  the  sake 
of  the  marked  advantage  of  spreading  more  chlorophyll;  and  in 
this  arrangement  we  see  one  of  those  innumerable  compromises 
with  which  plants,  like  mankind,  are  accustomed  to  meet  the  con- 
flicting problems  of  existence. 

Such  is  the  primary  or  ground  structure  of  stems,  as  typically 
displayed  in  their  earlier  stages,  and  up  to  the  time  when  they 
cease  to  be  flexible,  green  and  soft.  Then  they  begin  to  undergo 
remarkable  changes,  connected  adaptively  with  their  continuous 
growth  into  trees;  but  these  we  can  better  postpone  to  our  chapter 
on  Growth,  where  the  reader  will  find  them  fully  described. 

It  will  interest  the  reader  to  know  that  the  principal  theme  of 
this  chapter, — the  transfer  and  transpiration  of  water, — will  al- 
ways be  associated  in  the  minds  of  plant  physiologists  with  the 
foundation  of  their  science;  for  to  it,  of  all  the  phases  of  plant 
physiology,  was  first  applied  that  exact  scientific  method  of 
measurement  which  is  the  only  sure  means  for  advancing  natural 
knowledge.  Its  founder  was  Stephen  Hales,  whose  book  Vegetable 
Statics,  though  published  in  1727,  might  have  been  written' 
yesterday  so  far  as  its  spirit  is  concerned.  He  will  always  be 
considered  the  father  of  this  science,  and  his  book  one  of  the 
greatest  of  botanical  classics. 


CHAPTER  IX 

THE  PECULIAR  POWER  POSSESSED  BY  PLANTS  TO  ADJUST 
THEIR  INDIVIDUAL  PARTS  TO  THE  IMMEDIATE  SUR- 
ROUNDINGS 

Irritability 

F  the  reader  at  this  point  will  turn  back  to  the  Table 
displaying  the  plan  of  this  book,  he  will  see  that  we 
have  now  reached  the  end  of  our  survey  of  the  processes 
concerned  with  the  nutrition  of  plants.  These  proc- 
esses are  primarily  internal,  but  they  are  all  more  or  less  depend- 
ent, especially  for  their  supply  of  material  or  power,  upon  some 
one  or  the  other  of  the  external  conditions.  Now  these  external 
conditions, — heat,  light,  water,  minerals,  and  so  forth, — are  never 
distributed  quite  uniformly  around  any  individual  plant,  but  are 
more  or  less  abundant  in  some  spots  or  directions  than  others. 
Obviously  it  would  be  a  very  great  advantage  to  plants  if  each 
separate  one  of  their  parts, — each  leaf,  stem,  root,  and  so  forth, — 
could  be  adjusted  or  swung  individually  into  the  direction  or 
position  that  would  enable  it  to  work  to  the  very  best  advantage 
under  the  conditions  presented  by  its  own  immediate  surround- 
ings. Such  a  power,  and  in  high  degree  of  efficiency,  plants  in 
fact  do  possess,  as  we  shall  now  proceed  to  consider.  The  reader 
will  be  surprised,  I  predict,  by  the  importance  and  interest  of  the 
phenomena  which  belong  under  this  head. 

We  may  best  begin  our  study  of  the  subject  by  considera- 
tion of  its  most  familiar  example.  When  a  potted  plant,  like  a 
"Geranium,"  is  grown  in  a  greenhouse  lighted  evenly  all  around, 
it  assumes  a  symmetrical  form,  alike  on  all  sides,  as  everybody 

224 


Power  to  Adjust  Parts  to  Surroundings 


225 


knows;  but  when  the  same  plant  is  grown  in  the  window  of  a  room, 
where  the  light  is  wholly  one-sided,  it  turns  all  its  parts  in  that 
direction,  even  to  the  extent  of  seeming  to  reach  out,  as  it  were, 
after  the  light  (figure  74).  The  same  thing  occurs  commonly  in 
nature,  as  may  be  noticed  along  the  margin  of  shrubbery  or  close 


FIG.  74. — Two  "Geraniums"  which  for  two  or  three  days  before  their  pictures  were  taken, 
were  kept,  respectively,  in  a  uniformly  lighted  greenhouse  and  a  chamber  lighted  only 
from  the  right  hand  side. 

to  high  buildings  or  banks;  and  it  can  be  demonstrated  very 
prettily  by  experiment  (figure  75). 

A  close  observation  of  these  cases  shows  always  that  stems  and 
leaves  behave  very  differently  in  relation  to  the  direction  of  the 
light,  for  while  stems  point  straight  towards  it,  leaves  set  their 
faces  across  it.  This  suggests  the  inquiry, — what,  then  of  roots? 


226 


The  Living  Plant 


And  for  answer  we  turn  to  experiment.  If  seeds  of  mustard  or 
radish  are  started  in  water-culture  vessels,  by  methods  described 
in  an  earlier  chapter  (page  136),  the  young  seedlings  grow  rigidly 
upright  in  darkness;  but  if,  when  well  started,  they  are  given  a 


FIG.  75. — Sets  of  Radishes  grown  side  by  side  in  a  chamber  lighted  wholly  from  the  right 
hand  side;  but  those  on  the  instrument  were  kept  continually  revolving. 

one-sided  light,  they  turn  always  as  shown  in  our  figure, — the 
stems  to  the  light  and  the  leaves  across  it  as  before,  but  the  roots 
distinctly  away  (figure  76).  And  such  conduct  is  typical  of  or- 
dinary stems,  leaves  and  roots. 

This  process  of  light-turning  is  called  in  physiology  Photo- 


Power  to  Adjust  Parts  to  Surroundings  227 

tropism  (pronounced  with  the  accent  on  the  second  syllable),  or 

Hdiotropism.     Parts  that  turn  towards  light  are  described  as 

positively  phototropic    (with   the  accent,   despite  the  seeming 

anomaly,  on  the  third  syllable),  those  that  turn  away  as  negatively 

phototropic,   and   those    that    turn 

across  as  transversely  phototropic. 

Phototropism  is  so  thoroughly  typi- 

cal an  example  of  the  power  of  indi- 

vidual plant  parts  to  adjust  them- 

selves in  relation  to  the  immediate 

external  conditions  that  we  can  use 

it  as  a  basis  for  the  analysis  of  the 

nature  of  this  power,  which  is  known 

physiologically,    though    not    very 

happily,   as  Irritability.      Now  the 

elements  entering  into  irritable  re- 

sponses are  these:  — 

First,  the  reason  why  the  parts  do 
it.  —  As  to  this,  the  explanation 
must  be  amply  obvious.  The  turn- 
ing  towards  the  window  brings  the 
leaves  into  positions  where  they 
secure  the  best  exposure  to  light,  — 
the  light  which  is  indispensable  to  the  photosynthetic  func- 
tion for  which  they  exist.  The  best  position  for  performance 
of  this  function  must  of  course  be  that  which  sets  them  at 
right  angles  to  the  light;  and  this  in  turn  requires  that  the 
stem,  whose  function  is  simply  to  carry  the  leaves,  shall  point 
or  reach  towards  the  light.  As  to  the  roots,  not  only  does 
their  function  (the  absorption  of  water  and  minerals),  require 
no  light,  but  their  unprotected  protoplasm  is  actually  injured  by 
exposure  thereto  ;  and  this  shows  the  advantage  of  their  power  to 
retreat  from  light.  The  reason  for  the  characteristic  phototropism 
of  ordinary  leaves,  stems,  and  roots,  respectively,  is  therefore  to 


and  then  exposed  to  light  failing 

from  the  direction  of  the  arrow. 


f 


228  The  Living  Plant 

be  found  in  an  advantageous  functional  adjustment  of  those 
parts  in  relation  to  the  direction  of  light.  And  this  principle  of 
advantageous  individual  adjustment  of  parts  is  characteristic  of 
irritable  adjustments  in  general. 

Second,  the  mechanical  method  whereby  the  turning  is  effected. — 
The  turning  of  the  leaves,  stems,  and  roots 
into  their  respective  new  positions  requires 
both  a  considerable  power  and  a  definite 
mechanism.  Now  it  is  quite  evident  that  in 
phototropism  neither  of  these  is  supplied  by 
the  light,  for  that  has  no  power  at  all  to  lay 
bodily  hold  on  the  parts  and  forcibly  pull, 
bend,  or  push  them  into  their  respective 
positions,  while  it  is  easy  to  prove  on  the 
contrary  that  the  power  is  supplied  by  the 
plant,  and  derived  from  its  own  respiration. 
Thus,  if  oxygen  be  withdrawn  from  a  cham- 
ber in  which  a  symmetrical  plant  is  sub- 
jected to  one-sided  light,  not  a  trace  of 
phototropic  response  ever  follows.  The  con- 
nection will  be  clear  to  the  reader: — The 
FIG.  77.-Suecessive  stages  response  requires  energy,  energy  depends  on 
in  the  downward  turning  respiration,  respiration  demands  oxygen ; 

of  a  root,  showing,  by  the 

spread  of  the  marks,  that  therefore  no  oxygen,  no  response.  And  as 
Lhee/ectPeadreby  nTTftS  to  the  mechanism  of  the  turning,  that  also 
entiai  growth  and  not  by  -  ft  determined  by  experiment,  for  if 

the  bending  of  tissues  al-  •? 

ready  formed.    The  tn-  stems,  petioles,  or  roots  are  marked  across 

angular  piece  is  a  paper  j    v  i_    r  ji          i       .    • 

index.  (Copied  from  with  evenly-spaced  lines  before  the  plant  is 
exposed  to  one-sided  light,  then  the  marks 
spread  apart  in  a  way  to  prove  that  the  bending  accompanies 
growth  in  those  parts,  and  is  due  to  a  more  rapid  growth  on 
one  side  than  the  other, — on  just  that  side,  indeed,  where  it  is 
requisite  in  order  to  swing  the  parts  concerned  into  the  advan- 
tageous positions  (figure  77).  In  phototropic  adjustments, 


Power  to  Adjust  Parts  to  Surroundings  229 

therefore,  the  already-existent  tissues  are  not  forcibly  bent, 
but  the  new  tissues  grow  in  such  an  unequal  or  differential 
manner  as  to  swing  the  parts  into  their  new  positions.  In  these 
respects  phototropic  responses  are  typical  of  others,  for  in  all 
cases  the  power  is  supplied  by  the  responding  plant;  and  the 
motor  mechanism  consists,  as  a  rule,  in  such  differential  growth, 
though  occasionally  it  is  of  different  sort,  as  we  shall  presently 
note. 

Third,  the  way  the  light  operates  in  connection  with  the  turning. — 
Since  it  is  not  the  light  but  the  plant  which  accomplishes  the 
turning,  we  still  have  to  seek  the  nature  of  the  role  that  light 
takes  in  the  process.  In  brief,  observation  suggests  and  experi- 
ment proves  that  in  phototropic  responses  the  plant  parts,  which 
in  general  can  grow  quite  as  readily  in  one  direction  as  another, 
use  the  light  simply  and  solely  as  a  convenient  guide  or  signal 
(called  scientifically,  but  not  very  fortunately,  a  stimulus),  indica- 
tive of  the  most  advantageous  direction  to  take.  It  plays,  indeed, 
very  much  the  same  part  for  the  plant  that  the  compass  does  for 
the  sailor,  establishing  a  definite  line  of  direction,  towards, 
across,  or  from  which,  according  to  circumstances,  definite  move- 
ments may  be  made.  This  case  is  typical  of  the  action  of  stimuli 
in  general;  they  never  take  any  part  in  the  mechanical  accom- 
plishment of  the  irritable  adjustments,  but  serve  merely  as  signals 
for  guiding,  and  sometimes  for  starting  or  stopping,  the  same. 

Fourth,  the  way  the  light  stimulus  is  perceived  by  the  plant.— 
The  plant  has  no  eyes  for  the  light,  as  the  sailor  has  for  his  com- 
pass, yet  it  must  possess  some  means  of  perception  of  the  stimulus 
else  obviously  it  could  not  react.  The  details  of  the  matter  are 
still  much  in  doubt,  but  in  general  this  much  is  certain,  that  the 
light  falling  on  the  sensitive  protoplasm  of  the  plant  part  sets  up 
(probably  by  chemical  means,  since  the  blue  rays  are  mainly 
concerned)  a  condition  of  irritation  or  strain,  which  puts  the  side 
towards  the  light  in  a  condition  different  from  the  side  away 
from  it,  and  thus  establishes  the  line  of  light  direction.  This  case 


230  The  Living  Plant 

is  typical  of  all  stimuli,  which  act  by  producing  in  the  sensitive 
protoplasm  on  which  they  impinge  a  condition  of  differential 
irritation  or  strain  which  serves  to  impress  a  line  of  direction  on 
the  part  concerned.  Then  the  part  is  swung  by  the  motor  mech- 
anism into  a  position  where  this  condition  of  strain  is  the  same 
all  around,  which  position  is  kept  in  the  subsequent  growth.  Ob- 
viously only  those  agencies  can  act  as  stimuli  at  all  which  can 
thus  produce  a  differential  state  of  the  protoplasm,  and  con- 
versely, any  agency  capable  of  producing  such  a  condition  can, 
theoretically,  act  as  a  stimulus.  And  as  to  how  strong  a  stimulus 
must  be  to  produce  an  effect,  it  is  only  essential  that  it  have 
enough  power  to  produce  the  impression  of  differential  strain 
on  the  sensitive  protoplasm;  and  above  that  degree  its  strength 
does  not  much  matter. 

Fifth,  how  it  is  that  a  single  uniformly-acting  stimulus  can  evoke 
different  directions  of  turning. — The  fact  that  in  phototropism  the 
light  neither  pushes  nor  pulls  the  parts  to  their  positions,  but  acts 
simply  as  a  guide  to  direction,  involves  the  corollary  which  is 
confirmed  by  experience,  that  it  is  exactly  as  easy  for  parts  of  the 
plant  to  grow  away  from  or  across  the  light  as  towards  it,  pre- 
cisely as  the  sailor,  guided  by  his  compass,  which  neither  pushes 
nor  pulls  him  over  the  sea,  can  steer  as  easily  to  the  south,  east, 
or  west  as  to  the  north  where  it  points;  and  the  reader  should 
learn  to  think  of  all  stimuli  in  this  way.  But  if  the  parts  of  the 
plant  can  turn  as  easily  in  one  direction  as  another  in  relation  to 
light,  what  feature  of  their  growth-mechanism  is  it  which  sends 
stems  so  unerringly  towards  it,  leaves  across  it,  and  roots  from 
it?  Here  again  there  is  very  great  doubt  as  to  particulars,  but 
hardly  any  as  to  principle,  which  can  thus  be  illustrated.  In  a 
locomotive,  as  most  people  understand,  there  is  a  certain  lever, 
which  when  set  in  one  direction  determines  that  the  engine  shall 
move  forward,  and  when  set  in  another,  that  it  shall  move  back- 
ward, after  the  steam  is  turned  on;  and  an  engine  is  easily  imagi- 
nable in  which,  with  the  lever  in  yet  a  third  position,  the  move- 


Power  to  Adjust  Parts  to  Surroundings  231 

ment  would  be  sideways.  In  all  cases  it  is  the  same  engine,  the 
same  machinery,  the  same  motive  power;  the  difference  consists 
only  in  the  way  a  small  part  of  the  machinery  is  set;  and  the 
reader  will  please  to  observe  that  this  set  of  the  machinery  is  not 
the  cause  of  the  movement  of  the  engine,  but  merely  determines 
the  direction  thereof  when  the  power,  which  is  steam,  is  applied. 
Now  something  of  analogous  kind,  it  is  most  probable,  deter- 
mines the  direction  of  turning  of  the  plant  organs.  The  structure 
and  motive  power  in  all  of  these  parts  is  substantially  the  same, 
but  in  each  some  portion  of  the  machinery  is  differently  set,  so 
that  the  application  of  the  power,  which  is  growth,  causes  turn- 
ing in  the  distinctive  direction, — the  stem  towards  light,  leaf 
across  it,  and  root  from  it.  Of  course  the  machinery  is  not  metallic 
but  protoplasmic,  and  in  last  analysis  is  probably  of  a  chemical 
nature,  while,  moreover,  the  set  of  the  machinery  is  usually  not 
alterable  at  a  touch,  but  is  hereditarily  fixed  in  each  kind  of 
organ.  And  the  subject  may  stand  out  yet  more  clearly  if  we 
return  for  a  moment  to  our  sailor,  who,  in  order  to  reach  a  cer- 
tain eastern  port,  sets  his  steering  gear  to  hold  his  good  ship  at  one 
angle  to  his  compass,  and  in  order  to  reach  a  western  port  holds 
her  at  another.  It  is  the  same  compass,  ship,  machinery,  and 
power;  only  the  set  of  the  steering  gear  is  different.  This  is  the 
principle,  I  believe,  which  underlies  the  different  kinds  of  responses 
to  any  single  uniformly-acting  stimulus. 

Sixth,  how  the  advantageous  direction  of  response  has  become  fixed 
in  each  part. — Or,  in  the  simile  of  the  preceding  section,  how  did 
the  machinery  become  set  so  differently  in  leaf,  stem,  and  root; 
and  especially,  how  did  it  become  set  in  each  of  those  organs  in 
the  manner  most  advantageous  for  the  performance  of  its  particu- 
lar function?  Now  it  is  perfectly  plain  that  the  power  of  a  part 
to  respond  advantageously  to  a  stimulus,  that  is  to  say,  the  set 
of  its  responding  machinery,  is  an  hereditary  and  adaptive 
feature,  and  must  therefore  have  arisen  in  precisely  the  same 
manner  as  any  other  adaptive  features,  including  those  of  visible 


232  The  Living  Plant 

structure, — precisely,  for  example,  as  chlorophyll  has  been  de- 
veloped in  the  leaf,  a  fibre-vascular  cylinder  in  the  stem,  and 
hairs  on  the  roots.  Unless  our  whole  philosophy  of  nature  is 
wrong,  there  was  a  time  when  these  things  were  not:  now  they 
are :  at  some  time  and  in  some  way  meantime  they  have  arisen,  and 
by  gradual  stages  in  the  course  of  evolution.  Our  problem  of  the 
origin  of  the  set  of  the  machinery  is  therefore  identical  in  kind 
with  that  of  the  origin  of  any  adaptation,  and  thereby  is  trans- 
ferred into  that  separate  field  of  inquiry  which  forms  the  subject 
of  our  later  chapter  on  Evolution  and  Adaptation. 

The  turning  window-plant  illustrates  very  clearly  the  nature 
of  typical  sensitive  responses  in  plants;  and  all  of  the  more  com- 
plicated cases  are  identical  in  principle.  Thus,  not  all  stems 
turn  towards  light,  for  those  of  wall-climbing  Ivies  (e.  g.  the 
Boston  or  Japanese  Ivy)  turn  away  from  it,  as  manifest  by  the 
way  in  which  these  plants  grow  into  porches  and  windows.  The 
advantage,  however,  is  evident  on  reflection;  if  these  stems 
turned  towards  light,  like  the  ordinary  sort,  they  would  be  car- 
ried away  from  the  wall  and  the  possibility  of  clinging  thereto; 
but,  turning  away  from  the  light,  they  are  flattened  up  against 
the  wall  where  their  holding  discs  can  secure  an  attachment.  This 
example  shows  also  that  no  necessary  connection  exists  between 
sternness,  so  to  speak,  and  a  set  of  the  growth  machinery  towards 
light,  but  that  the  set  is  developed  in  the  organs  in  correlation 
with  their  habits  quite  regardless  of  their  morphological  nature. 
Again,  not  all  leaves  set  themselves  across  the  light,  for  a  good 
many  kinds  belonging  in  places  very  brilliantly  lighted,  like 
sub-tropical  plains,  set  their  edges  to  the  direction  of  maximum 
brightness.  In  some  this  position  is  permanent,  and  may  thus 
bring  the  leaves  to  a  vertical  north-and-south  position,  as  in  the 
Compass  Plant  of  our  prairies,  which  owes  its  name  to  this  cir- 
cumstance; or,  the  leaves  may  change  their  positions,  rising 
from  horizontal  to  vertical  at  the  time  of  maximum  brightness, 
as  in  sundry  plants  of  the  Pea  family  (figure  78).  The  advantage 


Power  to  Adjust  Parts  to  Surroundings  233 

of  these  vertical  light-positions  is  believed  to  consist  in  a  pro- 
tection given  to  the  living  substance  of  leaves  against  the  full 
exposure  to  a  brightness  too  intense  for  their  good;  for  we  know 
on  the  one  hand,  that  too  bright  a  light  does  chemical  damage  to 
protoplasm,  even  when  partially  screened 
by  the  chlorophyll,  while  on  the  other  hand, 
leaves  can  make  use  of  only  a  moderately 
strong  light,  the  extra  brightness  being 
wasted  upon  them.  It  is  this  last-mentioned 
circumstance,  by  the  way,  which  explains  a 
problem  that  sooner  or  later  will  puzzle  the 
reader,  viz.,  why  all  the  vegetation  in  the 
northern  hemisphere  does  not  have  a  turn 
towards  the  south  where  the  sun  is.  This 
is  no  doubt  because  the  diffused  light  falling 
on  the  plants  from  the  north  is  quite  as 
strong  as  they  can  use;  and  hence  they  have 
no  object,  so  to  speak,  in  turning  to  the  side 
of  the  sun. 

There  remains  one  other  phase  of  photot- 
ropism  in  leaves  which  must  here  be  consid-  Flo.78._^Uuitof  ^^ 
ered,  and  that  is  their  lateral  shiftings  out      tus>  showing  the  position 

assumed    in    the   bright 

from  beneath  one  another  s  shade,  a  move-  sun  by  the  leaflets  which 
ment  chiefly  accomplished  by  twisting  and  ^f  "co^Trom"* 
lengthening  of  the  petioles.  The  result  is  paper  by  w.  p.  Wilson.) 
often  to  bring  them,  especially  in  spread-out  plants  like  the 
vines,  into  a  one-planed  pattern  where  no  leaf  is  overlapped 
by  another, — an  arrangement  commonly  known  as  a  leaf-mosaic 
(figure  79) ;  and  there  are  even  some  botanists  who  believe  that 
the  angular  shapes  of  such  leaves  (e.  g.  in  the  English  Ivy)  are 
partly  determined  by  the  advantage  of  interlocking  to  use  all 
the  space. 

Such  lateral  shiftings  imply  that  the  whole  upper  surface  of 
the  leaf  is  equally  receptive  to  the  light  stimulus;  and  a  very 


234  The  Living  Plant 

ingenious  and  highly  probable  theory  has  been  advanced  in 
explanation,  viz.,  that  the  epidermal  cells,  focussing  the  light  in 
a  special  manner,  are  light-sensitive  organs,  and  that  the  leaf 
keeps  turning  and  shifting  until  all  of  these  cells  receive  their 
full  quota  of  light  at  the  most  desirable  angle.  In  some  other 
cases,  however,  the  reception  of  the  light  stimulus  is  known  to 
take  place  in  a  specialized  spot,  as  for  example  in  the  seedlings 
of  Grasses,  which  are  light-sensitive  only  in  the  tip  of  the  first 
sheathing  leaf.  The  same  thing  is  true,  for  several  stimuli,  of 
the  growing-point  of  the  root,  and  other  cases  are  known.  Evi- 
dently some  such  structures  advance  pretty  far  in  the  direction 
of  the  special  sense  organs  of  animals,  such  as  eyes.* 

Thus  much  for  the  phototropism  of  stems,  leaves,  and  roots: 
what  now  of  flowers  and  fruits?  As  to  flowers,  they  turn  their 

*  The  localized  reception  of  stimuli  by  the  growing  points  of  the  roots  is  strikingly 
expressed  by  Darwin  in  the  closing  paragraph  of  his  great  book,  The  Power  of  Move- 
ment in  Plants;  and  this  passage  illustrates  so  well  a  number  of  other  phases  of 
irritable  responses  that  it  is  here  reprinted  in  full. 

"We  believe  that  there  is  no  structure  in  plants  more  wonderful,  as  far  as  its 
'functions  are  concerned,  than  the  tip  of  the  radicle.  If  the  tip  be  lightly  pressed 
'or  burnt  or  cut,  it  transmits  an  influence  to  the  upper  adjoining  part,  causing  it 
'to  bend  away  from  the  affected  side;  and,  what  is  more  surprising,  the  tip  can 
'distinguish  between  a  slightly  harder  and  softer  object,  by  which  it  is  simultane- 
'ously  pressed  on  opposite  sides.  If,  however,  the  radicle  is  pressed  by  a  similar 
'object  a  little  above  the  tip,  the  pressed  part  does  not  transmit  any  influence  to 
'the  more  distant  parts,  but  bends  abruptly  towards  the  object.  If  the  tip  per- 
'  ceives  the  air  to  be  moister  on  one  side  than  on  the  other,  it  likewise  transmits  an 
'influence  to  the  upper  adjoining  part,  which  bends  towards  the  source  of  moisture. 
'  When  the  tip  is  excited  by  light  (though  in  the  case  of  radicles  this  was  ascertained 
'in  only  a  single  instance)  the  adjoining  part  bends  from  the  light;  but  when  excited 
'  by  gravitation  the  same  part  bends  towards  the  center  of  gravity.  In  almost  every 
'case  we  can  clearly  perceive  the  final  purpose  or  advantage  of  the  several  move- 
'  ments.  Two,  or  perhaps  more,  of  the  exciting  causes  often  act  simultaneously  on  the 
'  tip,  and  one  conquers  the  other,  no  doubt  in  accordance  with  its  importance  for  the 
'life  of  the  plant.  The  course  pursued  by  the  radicle  in  penetrating  the  ground 
'must  be  determined  by  the  tip;  hence  it  has  acquired  such  diverse  kinds  of  sensi- 
'tiveness.  It  is  hardly  an  exaggeration  to  say  that  the  tip  of  the  radicle  thus  en- 
'  do  wed,  and  having  the  power  of  directing  the  movements  of  the  adjoining  parts, 
'acts  like  the  brain  of  one  of  the  lower  animals;  the  brain  being  seated  within  the 
'  anterior  end  of  the  body,  receiving  impressions  from  the  sense-organs,  and  directing 
'the  several  movements." 


Power  to  Adjust  Parts  to  Surroundings  235 

faces,  as  a  rule,  directly  to  the  light  like  the  leaves,  as  anyone 
can  observe  in  our  house  plants,  or  in  those  that  happen  to  grow 
close  to  a  building  (e.  g.  a  border  of  Nasturtiums),  or  against 
walls  (e.  g.  Trumpet  Creeper),  or  otherwise  in  one-sided  light 
(figure  80).  In  a  few  flowers  (e.  g.  Sunflowers),  the  phototropism 
even  extends  to  the  following  of  the  sun  through  the  day,  though 
the  adjustment  is  only  moderately  effective.  Perhaps  at  first 
thought  it  will  not  be  evident  why  flowers  are  phototropic  at  all, 


FIG.  79. — The  adjustment  of  Ivy  leaves  (of  English  Ivy)  into  one  plane,  affording  the  best 
aggregate  exposure  to  light.     (Copied,  reduced,  from  Kerner's  Pflanzenleben.) 

because,  unlike  the  leaves,  there  is  nothing  in  the  function  of  the 
flower  requiring  the  action  of  light.  But  on  further  contempla- 
tion of  the  use  of  the  flower  (a  subject  to  be  fully  explained  in 
the  chapter  upon  Cross-pollination),  and  especially  of  the  function 
of  the  showy  corolla  as  an  advertisement  to  show  insects  its 
position,  the  matter  becomes  evident;  because  obviously  this 
function  of  conspicuousness  requires  that  the  corolla  must  stand 
out  where  the  light  can  strike  on  it  most  fully.  As  to  fruits,  they 
are  as  a  rule  indifferent  to  light,  though  responsive  to  some 
other  kinds  of  stimuli,  as  will  later  appear.  One  special  case, 
however,  deserves  mention  because  illustrative  of  an  additional 
fact  about  stimuli.  There  grows  in  Europe  a  little  cliff-dwelling 
vine,  Linaria  Cymbalaria  (figure  81),  which  turns  its  flowers  as 
usual  to  the  sun,  but  its  ripening  seed-capsules  away  therefrom. 


236 


The  Living  Plant 


In  consequence  these  seed  capsules  are  brought  into  contact  with 
the  cliff,  and,  moving  about  more  or  less,  are  reasonably  sure  to 
push  into  some  crevice  where  the  seeds  can  be  dropped  in  posi- 
tion for  starting  the  new  plant 
in  its  favorite  habitat,  instead 
of  at  the  foot  of  the  cliffs. 
There  are  two  good  reasons 
why  I  cite  this  example.  In  the 
first  place  it  shows  that  the 
phototropism  of  a  part  may 
change  —  the  lever  may  be 
thrown — during  its  own  life, 
though  this  is  not  common. 
In  the  second  place,  the  seed 
capsule  has  obviously  no  need 
to  get  away  from  the  light  as 
such,  but  simply  to  get  back 
against  the  cliff.  Since,  how- 
ever, there  exists  no  cliff-ward 
stimulus,  the  light,  which  hap- 
pens to  act  in  the  suitable  di- 
rection, is  used  for  the  purpose. 
Light  in  this  case  acts  as  a 
foster-stimulus  as  it  were,  and 
may  thus  be  described,  in  con- 
trast with  the  direct  stimuli  of 
the  examples  earlier  described. 
There  remains  one  other  class 

FIG.  80. — A  cut  shoot  of  Bellflower,  kept  for        /•  r    -,.  ,-,  H    j 

two    days  in   a    chamber   lighted  wholly     of  light  responses,— the  SO-Called 
from  the  left.     Observe  the  positive  pho-     sjeep    movements. 


totropism  of  the  flowers. 


It  is  very 

well  known  that  some  leaves 
droop  at  night,  as  in  Clovers,  Wood-sorrels,  Beans,  and  many 
other  members  of  the  Pea  family  (figure  82);  and  most  people 
have  seen,  at  some  time  or  other,  the  remarkably  tight-shut  ap- 


Power  to  Adjust  Parts  to  Surroundings  237 

pearance  presented  by  those  plants  at  night.  The  same  plants, 
moreover,  can  be  put  to  sleep  very  easily,  even  at  midday,  by 
simply  covering  them  up  from  the  light.  Now  the  exact  meaning 
of  the  sleep  movement  is  somewhat  in  doubt,  as  our  chapter  on 
Protection  will  show;  but  there  is  no  question  at  all  that  light  is 
the  stimulus  concerned.  This  response  has,  however,  an  interest 
in  another  direction,  for  the  motor-mechanism  is  not  growth,  but 
a  simple  hydraulic  contrivance  contained  in  the  clear  little 
swellings  at  the  bases  of  the  sleeping  leaflets.  In  the  daytime, 


FIG.  81. — The  cliff-dwelling  plant,  Linaria  Cymbalaria,  showing  the  positive  phototropism 
of  its  flowers  and  the  negative  phototropism  of  its  seed  capsules,  which  thus  are 
brought  Into  advantageous  positions  for  the  deposition  of  the  seeds.  (Copied,  sim- 
plified, from  Kerner's  Pflanzenleben.) 

under  stimulus  of  light,  their  cells  become  strongly  turgescent 
and  hold  the  leaves  stiffly  expanded ;  but  at  night  the  turgescence 
is  lessened,  and  the  spring  of  the  tissues,  aided  more  or  less  by 
gravitation,  causes  them  to  droop.  It  is  perhaps  simply  a  high 
degree  of  development  of  sleep  movement  which  gives  us  the 
remarkably-balanced  leaf  mechanism  of  the  Sensitive  Plant, 
later  to  be  considered. 

In  viewing  these  sensitive  responses,  and  others  of  similar 
sort,  one  soon  comes  to  wonder  what  the  limits  may  be  to  the 
changes  they  can  cause  in  the  construction  of  the  plant.  This, 
like  most  of  our  problems,  is  amenable  to  experiment.  If  the 
most  favorable  possible  conditions  for  one-sided  stimulation  are 


238 


The  Living  Plant 


supplied  to  a  plant,  it  will  turn  to  that  side  to  a  considerable 
degree;  but  the  turning  is  never  without  limit,  for,  generally 
speaking,  the  farther  it  turns  the  more  reluctant,  so  to  speak, 
it  is  to  turn  any  farther.  If,  on  the  other  hand,  the  plant  is  so 
grown  that  it  does  not  receive  a  one-sided  stimulation,  which  is 


FIG.  82. — A  typical  example  of  the  sleep  of  plants.    Both  are  Acacias,  identical  in  kind  arid 
age,  but  the  one  on  the  right  has  been  covered  for  an  hour  from  light. 


easiest  accomplished  by  keeping  the  plant  in  continual  rotation 
by  aid  of  an  instrument  (called  a  clinostat)  designed  for  the 
purpose  (figure  83),  then  it  always  develops  with  remarkable 
symmetry,  determined,  very  obviously,  by  internal  and  hereditary 
causes.  The  plant,  accordingly,  is  born  with  an  internal  tendency 
to  symmetrical  form,  but  likewise  with  a  considerable  though  not 
unlimited  margin  of  possible  deviation  therefrom;  and  it  is 
within  this  margin  that  the  irritable  responses  take  place.  But 


Power  to  Adjust  Parts  to  Surroundings 


239 


this  margin  has  a  greater  interest  than  this,  for  it  is  characteristic 

of  animals  also,  including  ourselves,  where  it  offers  the  basis  for 

improvement    of    the    body 

through  exercise,  and  of  the 

mind      through     education, 

while  it  is  the  field,  as  well, 

within  which  plays  such  free- 

dom as  is  possessed  by  the 

will. 

Phototropism  has  received 
this  generous  measure  of 
attention  because  it  is  so 
thoroughly  typical  of  irri- 
table responses  in  general. 
Accordingly  the  remaining 
forms  of  irritability  can  be 
treated  much  more  briefly. 

Hydrotropism.  —  If  one  pre- 
pares a  porous  clay  germina- 
tor  of  the  cylindrical  form 
represented  in  our  picture 
(figure  84)  :  fills  it  with  water  : 
hangs  it  horizontally  :  fastens 
small  seeds  along  its  sides: 
and  places  it  in  a  chamber 
with  a  vapor-saturated  at- 
mosphere, then  the  stems 

and  the  roots  Will  grOW  Stiff-    FlG-   83-—  The  clinostat,   an  instrument   which 

allows  the   effect  of  one-sided  stimuli  to  be 
ly    Up    and    down     as    Shown 

Dy    the    first    OI    the     figures. 

But  if  the  surrounding  air 
be  partially  dry,  then  the  roots  will  cling  close  to  the  porous 
and  water-soaked  germinator,  though  the  stems  will  act  precisely 
as  before.  In  the  first  case  the  moisture  is  the  same  all  around; 


neutralized  through  the  continual  slow  rota- 
tion  of  the  plant.  Note  the  resultant  sym- 
metry  of  the  Nasturtium  which  has  been 


240 


The  Living  Plant 


in  the  second  it  is  most  abundant  on  the  side  towards  the  ger- 
minator.  The  experiment,  therefore,  shows  that  roots  turn  in  the 
direction  where  moisture  is  most  plenty; — that  is,  they  possess  a 
definite  hydrotropism,  another  typical  form  of  irritable  response. 
The  advantage  of  hydrotropism  is  perfectly  evident  when  one 
recalls  that  the  very  first  function  of  roots  is  the  absorption  of 


FIG.  84. — Porous  water-filled  cylinders,  to  which  seeds  of  Mustard  were  attached.    That 
on  the  left  was  then  kept  in  a  saturated,  and  that  on  the  right  in  a  drier,  atmosphere. 

water.  The  stimulus  acts  in  this  way;  the  water,  absorbed 
more  rapidly  on  the  side  of  its  greatest  abundance,  doubtless 
causes  an  osmotic  swelling  and  tension  stronger  on  that  side 
than  on  the  other;  and  this  difference  is  ample  to  establish  a  line 
of  direction  towards  which  the  roots  turn  in  their  growth.  It  is 
equally  easy  to  see  why  stems  and  leaves  display  no  hydrotro- 
pism at  all,  for,  as  they  do  not  absorb  any  water  under  normal 


Power  to  Adjust  Parts  to  Surroundings  241 

conditions,  its  one-sided  abundance  is  a  matter  of  indifference 
to  them.  This  fact  illustrates  anew  the  adaptive  character  of 
these  responses;  for  it  is  a  general  rule  that  plant  parts  are  in- 
different to  stimuli  to  which  there  is  no  profit  in  responding. 

The  hydrotropism  of  roots  involves  matters  of  some  practical 
consequence.  It  is  said  that  when  trees  develop  in  a  uniform 
soil,  the  root  tips  tend  to  collect  in  a  circle  just  under  the  outer 
drip  of  the  foliage,  which  is  obviously  the  place  where  the  water 
is  usually  most  plenty.  But  in  case  the  soil  is  moister  on  one 
side  than  another,  the  roots  grow  more  freely  in  that  direction, 
and  may  even  extend  to  a  distance  several  times  the  diameter  of 
the  tree.  In  their  progress  thus  towards  the  most  copious  wet- 
ness, they  sometimes  are  led  to  a  drain,  and,  insinuating  them- 
selves through  some  crevice  left  in  the  tiles,  find  therein  a  com- 
bination of  water,  air,  and  mineral  substances  so  agreeable  that 
they  grow  very  profusely,  even  to  so  great  a  degree  that  they 
sometimes  choke  the  drain  quite  completely. 

Chemotropism. — But  roots  have  also  other  irritable  responses, 
notably  to  some  chemical  substances.  Thus  they  turn,  though 
rather  feebly,  towards  a  source  of  supply  of  some  of  the  minerals 
they  absorb,  and  this  is  typical  chemotropism,  with  a  very  ob- 
vious advantage.  But  they  turn  much  more  strongly  towards 
air  (a  special  phase  of  chemotropism  called  AEROTROPISM), — of 
course  for  the  oxygen  it  contains,  which  they  need  for  their 
respiration.  It  is  easy  to  see  in  these  cases  how  the  stimulus  is 
received  by  the  root,  for  the  chemical  substance,  especially  the 
oxygen,  must  react  with  some  of  the  materials  found  in  the 
complicated  protoplasm  with  which  it  first  comes  into  contact, 
thus  originating  a  differential  chemical  disturbance  which  would 
establish  the  line  of  direction. 

But  other  structures  besides  roots  are  markedly  chemotropic. 
Thus  pollen-tubes  in  their  growth  turn  towards  the  substances 
secreted  by  stigmas  and  styles.  In  the  fertilization  of  Ferns,  an 
egg-cell  at  the  bottom  of  a  protective  flask-like  archegonium  is 


242  The  Living  Plant 

fertilized  by  a  male  antherozoid  which  swims  through  the  water 
(figure  104).  Now  when  this  egg-cell  is  ready  for  fertilization,  a 
weak  solution  of  malic  acid  pours  out  of  the  archegonium  into  the 
water,  and  diffuses  steadily  outwards.  As  soon  as  some  wandering 
antherozoid  perceives  the  presence  of  the  acid,  it  turns  and  swims 
directly  towards  the  source  of  supply,  and  hence  to  the  egg-cell, 
which  otherwise  it  would  have  no  means  to  discover.  And  there 
is  reason  to  think  that  such  a  secretion  of  special  chemicals  at  the 
time  when  the  egg-cells  are  ripe  is  very  wide  spread  through  the 
plant  and  animal  kingdoms,  providing  the  method  whereby  the 
swimming  or  growing  male  cells  are  enabled  to  find  the  female 
cells.  This  function  is  obviously  not  simply  advantageous  but 
indispensable. 

There  are  many  important  phases  of  chemotropism,  but  I 
have  the  space  to  mention  only  one  more.  Water-plants,  which 
have  floating  leaves,  alter  the  lengths  of  the  petioles  in  accord- 
ance with  the  depth  of  the  water,  a  matter  which  can  be  shown 
very  beautifully  by  experiment.  Now  it  is  found  that  this  regula- 
tion is  chemotropic,  or,  more  exactly,  aerotropic,  for,  as  ex- 
periment proves,  petioles  continue  to  grow  until  the  leaves 
reach  a  supply  of  free  oxygen,  when  they  stop.  This  case  illus- 
trates an  additional  fact  about  stimuli,  viz.  that  they  can  serve 
as  signals  to  stop  a  process  as  well  as  to  guide  it ;  and  other  cases 
are  known  in  which  they  act  to  start  a  process.  Such  stimuli 
are  probably  very  important  in  controlling  the  various  processes 
of  growth,  as  our  later  chapter  on  that  subject  will  demonstrate. 

Thigmotropism. — This  name  is  applied  to  those  turnings  and 
movements  made  in  response  to  a  touch  as  a  stimulus.  The 
most  typical  case  is  exhibited  by  tendrils,  which,  as  the  reader 
will  recall,  are  those  long  slender  structures  sent  reaching  out  for 
a  support  by  a  good  many  kinds  of  climbing  plants.  These 
tendrils  sweep  in  long  slow  courses  through  the  air  until  they 
touch  some  hard  object,  such  as  a  stem,  or  a  wire,  around  which 
they  then  curl  in  three  or  four  turns  (figure  85),  thus  obtaining 


Power  to  Adjust  Parts  to  Surroundings  243 

a  grip  which  holds  the  vine  firmly  and  permits  a  still  farther 
ascent.  Now  it  is  easy  to  prove  by  experiment  that  it  is  really 
the  contact  with  the  support  which  constitutes  the  stimulus 
producing  the  bending,  for  anyone,  by  rubbing  one  side  of  a 
tendril  with  a  pencil,  can  call  out 
the  turning,  and  watch  all  of  the 
steps  in  its  progress.  Even  a  mo- 
mentary contact  is  followed  by 
a  turning  within  a  few  minutes, 
though  the  tendril  will  straighten 
again  in  case  the  contact  is  not 
maintained;  but  if  the  contact  be 
continuous  the  tendril  will  wind 
completely  around  the  pencil.  The 
advantage,  the  motor-mechanism 
(which  is  growth),  and  the  mode  of 
reception  of  the  stimulus,  in  this 
form  of  thigmotropism,  are  all  suf- 
ficiently obvious. 

Most  persons  who  have  knowl- 
edge of  plants  would  doubtless  put 
forward  a  different  case  as  a  type 
of  thigmotropism,  viz.,  the  well- 
known  Sensitive  Plant,  which  droops 
promptly  and  completely  at  a  touch 

(figure  86).       But  I  think  this  move-    FIG.    85.— Four    successive   stages    in 

ment  is  only  accidentally  thigmo- 


tropic.        Nobody     has     yet     found,        Plified-  from  a  wall-chart  by  Lau- 
rent and  Errera.) 

even  after  study  of  the  plant   in 

its  native  home,  any  satisfactory  reason  why  the  plant  should 
droop  for  a  touch,  while,  on  the  other  hand,  it  responds  in 
the  same  manner  to  other  kinds  of  stimuli, — a  scorch  of  flame, 
a  strongly-focussed  light,  a  trifle  of  acid — to  which  there  can 
be  no  question  of  adjustment.  The  leaves  have,  however, 


244 


The  Living  Plant 


yet  one  other  marked  response,  and  that  most  important,  be- 
cause, as  is  probable,  it  explains  the  original  adaptation, — 
viz.  a  marked  sleep  movement  just  like  those  which  we  have 
noticed  already  under  phototropism.  The  motor-mechanism 
underlying  the  droop  of  the  leaves  of  the  Sensitive  Plant  is  a 


FIG.  86. — Two  Sensitive  Plants,  of  which  the  one  on  the  right  was  struck  a  sharp  blow 
just  before  the  photograph  was  taken. 

particularly  efficient  example  of  the  hydraulic  type  already  men- 
tioned; and  probably  it  is  so  highly  perfected  and  delicately- 
balanced  that  although  developed  originally  in  connection  with 
sleep  movements,  it  can  now  be  set  off,  so  to  speak,  by  various 
other  stimuli,  such  as  touch, — precisely,  for  example,  as  a  cannon 
can  be  fired  by  a  lighted  match,  an  electric  current,  some  chemi- 
cals, or  a  sharp  blow.  The  sensitiveness  of  the  Sensitive  Plant  to 
touch  is  upon  this  explanation  accidental ;  and  there  are  probably 
yet  other  examples  of  such  accidental  stimulation  in  other  phases 


Power  to  Adjust  Parts  to  Surroundings  245 

of  irritability.  Indeed,  in  the  very  highly  complicated  and  un- 
stable organization  of  the  plant,  it  must  often  happen  that  the 
motor  or  growth  mechanisms  are  set  off,  quite  accidentally, 
by  various  wholly  unrelated  stimuli.  Such  is  undoubtedly  the 
nature  of  many  of  the  "mechanical  responses,"  which  by  some 
recent  writers  have  been  made  the  basis  of  all  plant  activities, 
development  and  evolution,  quite  regardless  of  the  innumerable 
other  elements  and  conditions  entering  into  the  constitution  of 
organisms. 

A  good  many  additional  cases  of  thigmotropic  irritability  are 
known.    Thus,  the  leaves  of  some  Insectivorous  Plants  close  upon 


FIG.  87. — Corn  seedlings,  showing  the  uniformity  of  position  assumed  by  the  growing  roots 
and  stems,  respectively,  from  very  diversely  placed  seeds. 

flies  that  alight  upon  them, — quickly  in  the  Venus  Fly-trap,  and 
slowly  in  Sundew.  Some  stamens  when  touched  by  insects,  move 
up  in  such  a  way  as  to  dust  those  visitors  thoroughly  with  pollen, 
thus  aiding  in  the  utilization  of  insects  for  cross-pollination  of 
flowers,  of  which  the  importance  will  later  become  apparent  to 
the  reader.  In  these  and  some  analogous  cases,  the  advantage, 
mechanism,  and  method  of  stimulation  are  all  more  or  less  well 
understood. 

Geotropism. — When  seeds  fall  to  earth,  or  are  placed  in  the 
ground  by  a  gardener,  they  come  to  rest  in  the  most  diverse 
positions,  with  their  embryonic  roots  and  stems  pointing  at  any 
and  all  angles.  Nevertheless,  as  they  germinate,  the  young 
roots,  with  a  singular  unanimity,  turn  downwards  and  the  stems 
upwards.  The  same  thing  can  be  shown  very  clearly  by  ex- 


246  The  Living  Plant 

periment,  for  if  a  number  of  large  seeds,  such  as  Windsor  Beans, 
or  Corn,  be  fixed  in  the  most  diverse  possible  positions  (figure  87), 
the  new  stems  and  roots  will  grow  themselves  round  into  the 
up-and-down  directions  respectively.  Furthermore,  the  side 

roots  as  they  come  out,  and  side 

•  v  J>/  branches  as  well,  assume  and  hold 

for  a  time  a  definite  angle  to  the 
same  up-and-down  line.  That  the 
positions  of  these  parts  are  taken 
with  reference  to  the  up-and-down 
line,  and  not  simply  in  relation 
to  the  main  root  and  stem,  is 
proven  by  a  very  conclusive  ex- 
periment; for  if  the  young  plants, 
when  their  parts  are  well  formed, 
are  tipped  over  at  an  angle,  or  up- 

FIG.  88.— This  Bean  seedling  was  grown  side  down  as   shown  by  Olir   figure 

for  a  time  in  this  position;  then  it  was  ._                     N        ,              n        .     , 

inverted,  and  the  new  growth  is  repre-  (ngUre    OOj,    then    all    OI    the    parts 

sented  in  solid  black;  finally  it  was  re-  <__„„,   ,..„   nnipL-1-.r  Po   fV>PV  r>nn    intn 

turned  to  its  first  position  and  has  made  8rOW  aS   qmcK:1y  as  tneY  Can   mto 

still  further  growth.   The  direction  of  their   former   directions.     A  case 

growth    is    obviously    geotropic,    not 

relative  to  the   main  root.      (Copied,   of     analogOUS    SOrt    IS    found    also 

reduced,  from  Sachs'  Lectures.)  AT  , 

in  Nature,  where  evergreen  trees 

that  grow  on  irregular  steep  hillsides  show  no  relation  what- 
ever to  the  slope  of  the  ground,  but  grow  as  stiffly  upright, 
and  with  branches  as  truly  horizontal,  as  if  the  ground  were 
quite  level.  These  simple  illustrations  are  typical  of  a  well- 
nigh  universal  fact  about  plants, — that  they  send  their  first 
roots  down  and  their  first  stems  up,  and  their  side  roots  and 
side  stems  out  at  definite  angles  to  the  up-and-down  direc- 
tion, regardless  of  the  conditions  under  which  they  originate. 
This  fact  is  fundamental  in  the  economy  of  vegetation,  for  it 
helps  to  explain  the  way  in  which  large  plants  can  guide  their 
growth  into  upright  positions,  and  hold  themselves  therein, 
and  how  they  can  spread  out  their  branches  at  such  definite 


Power  to  Adjust  Parts  to  Surroundings  247 

angles  as  to  give  to  these  plants  their  characteristic  outlines. 
Furthermore  it  also  explains  how  stems  can  so  readily  recover 
their  natural  positions  when  the  plants  are  over-turned,  whether 
by  accident,  or  by  intention  in  experiment. 

We  must  next  turn  attention  to  this  crucial  matter  of  the 
up-and-down  line.  Now  there  is  in  this  world  only  a  single 
determinant  thereof,  and  that  is  the  attraction  of  gravitation, 
which  forever  is  drawing  all  objects  towards  the  center  of  the 
earth.  Gravitation,  therefore,  would  seem  to  be  the  stimulus 
used  by  the  plant  in  assuming  the  positions  we  are  considering. 
In  other  words,  the  parts  of  the  plant  are  geotropic ; — and  all  evi- 
dence confirms  this  conclusion. 

The  wide  use  of  gravitation  as  a  stimulus  raises  at  once  the 
question  as  to  the  physiological  value  of  gravitation  to  the  plant. 
In  itself,  however,  it  has  no  value,  so  far  as  anyone  has  been  able 
to  discover.  The  plant  has  no  object  at  all  in  sending  roots 
downward  and  shoots  upward  merely  to  have  them  down  and 
up;  but  it  happens  that  down  is  the  direction  of  moisture  and 
minerals,  which  roots  need,  and  up  is  the  direction  of  light,  which 
shoots  need.  No  doubt  those  parts  could  be  guided  in  the  need- 
ful directions  by  then*  hydrotropism  and  phototropism  respect- 
ively, but  gravitation  has  this  advantage  over  moisture  and 
light  as  a  stimulus,  that,  while  happening  to  act  in  the  suitable 
direction,  it  is  present  unvaryingly  at  all  times,  whereas  light 
and  moisture  are  most  variable  in  quantity,  and  sometimes 
absent  altogether.  This  is  especially  true  of  light,  which  is  missing 
at  night  when  growth  is  most  active  and  the  guiding  stimulus 
most  needed.  Gravitation,  therefore,  is  neither  a  direct,  nor  a 
foster  stimulus,  like  those  we  have  already  considered,  but  a 
substitute  stimulus,  adopted  by  the  plant  in  place  of  other 
stimuli  because  it  acts  better  than  they.  The  use  of  the  compass 
has  just  the  same  advantage  over  observation  of  the  sun  and  the 
stars,  which  would  also  take  the  sailor  to  his  port ;  for  the  compass 
is  constant  in  its  action,  while  the  sun  and  the  stars  not  only 


248  The  Living  Plant 

vary  in  direction  all  through  the  twenty-four  hours,  but  often- 
times are  obscured  altogether.  Moreover,  this  principle  of  sub- 
stitution stimuli  is  often  important  in  connection  with  the  de- 
velopment of  structures,  for  it  helps  to  explain  how  an  organ  or 
other  feature  can  form  in  advance  of  perception  of  the  stimulus 
to  which  it  is  later  to  react, — e.  g.  the  formation  of  the  eye  before 
birth  in  animals,  and  of  chlorophyll  in  the  embryos  of  plants. 

The  way  in  which  the  gravitation  stimulus  is  perceived  by  the 
plant  seems  clear.  Gravitation  draws  the  heavier  contents  of 
the  cells,  especially  the  starch  grains,  down  to  the  bottom  of  the 
cell,  where  their  weight  presses  hard  on  the  sensitive  protoplasm 
and  produces  a  condition  of  strain  different  from  anything  in  the 
upper  part  of  the  cell;  and  this  difference  establishes  the  line  of 
direction.  Then  the  responding  mechanism  is  so  set  that  main 
roots  are  sent  growing  towards  this  pressure,  main  stems  away 
from  it,  and  side  parts  across  it,  precisely  as  in  other  typical 
responses.  Geotropism,  by  the  way,  is  a  perfect  illustration  of 
the  fact  that  a  stimulus  acts  merely  as  a  guide,  and  not  as  a 
physical  aid,  to  responses;  for  while  gravitation  might  be  sup- 
posed to  help  pull  roots  downward,  obviously  it  cannot  be 
imagined  to  help  push  stems  upward  or  to  drive  side  parts  out 
crossways. 

Thus  much  for  the  geotropism  of  stems  and  roots;  what  of 
leaves,  flowers  and  fruits?  As  to  leaves,  their  geotropism  is 
usually  disguised  by  their  stronger  phototropism;  but  that  they  are 
geotropic  is  shown  by  the  vertical  or  horizontal  positions  they 
assume  when  kept  in  dark  rooms.  We  see  another  illustration 
thereof,  as  I  take  it,  in  Nature,  in  some  of  the  broad-leaved 
shrubs  which  grow  in  the  shade  of  the  forest;  here  the  diffused 
light  is  so  evenly  distributed  that  it  exerts  no  one-sided  stimulus, 
and  the  leaves  are  left  free  to  assume  their  geotropic  position, 
which  is  strikingly  horizontal.  As  to  flowers,  they  also,  for  the 
most  part,  are  definitely  geotropic.  Thus,  if  one  selects  a  long 
terminal  cluster  of  unopened  irregular  flowers,  such  as  Larkspur 


Power  to  Adjust  Parts  to  Surroundings 


249 


or  Snapdragon,  bends  it  over  and  fastens  it  down  at  the  tip,  as 
shown  by  our  figure  (figure  89),  then  each  of  the  blossoms,  as  it 
opens,  turns  over  individually  to  the  very  same  position  it  would 


FIG.  89. — Flower  shoots  of  Larkspur,  the  curved  one  of  which  was  bent  over  and  fastened 
a  few  days  before  this  picture  was  taken.  Note  the  uniform  geotropic  positions  as- 
sumed by  both  buds  and  flowers. 

have  had  in  the  vertical  cluster.  The  position  of  each  separate 
flower  is  here  established  geotropically,  and  for  a  very  good  rea- 
son,— viz.,  these  irregular  flowers,  as  our  later  chapter  on  the 


250  The  Living  Plant 

subject  will  show,  are  specialized  for  cross-pollination  in  a  way 
which  makes  the  lowermost  petals  alighting  places  for  insects; 
and  therefore  these  petals  must  be  kept  horizontal.  For  the 
same  reason  the  long  tubes  of  Daffodils  are  geotropically  hori- 
zontal, as  one  can  prove  by  fastening  the  young  flower-stems  in 
horizontal  positions;  and  there  are  other  cases  without  number. 
As  to  fruits,  they  are  mostly  indifferent  geotropically,  but  a  few, 
e.  g.  Cyclamens  and  Pea-nuts,  use  gravitation  as  a  guide  as  they 
bury  their  seeds  in  the  earth. 

So  many  and  interesting  are  the  manifestations  of  geotropism 
in  special  cases  that  I  must  take  room  for  a  few  more  examples. 
Trailing  vines,  whose  main  stems  rest  flat  on  the  ground,  like  the 
Periwinkle,  Twin-flower,  and  Ground  Pine,  and  perennials  with 
horizontal  stems  just  beneath  it,  like  Solomon's  Seal,  keep  these 
positions  by  virtue  of  the  fact  that  their  main  stems  have  not  the 
usual  main-stem  geotropism,  which  is  upright,  but  the  trans- 
verse kind  characteristic  of  side-branches;  twining  plants  are 
kept  encircling  a  vertical  support  under  guidance  of  a  lateral 
geotropism,  and  this  is  what  prevents  them  from  twining  around 
horizontal  branches  or  supports  which  would  not  take  them  up 
towards  the  light ;  the  aerial  roots  of  many  tropical  climbers,  and 
most  tendrils,  have  likewise  this  lateral  geotropism,  which  keeps 
them  swinging  horizontally  until  they  meet  with  a  support;  and 
there  are  many  other  cases  of  which  some  may  be  identified  by 
the  reader  himself  if  he  keeps  observationally  alert  in  his  walks 
abroad  in  field,  garden,  or  forest. 

Of  all  of  the  stimuli  made  use  of  by  plants  for  guiding  their 
parts  to  positions  of  greatest  advantage,  gravitation  is  much  the 
most  important.  Plants  are  born  with  an  hereditary  tendency 
to  put  forth  their  parts  in  a  symmetrical  manner,  as  can  be 
demonstrated  experimentally  by  aid  of  the  clinostat;  but  they 
depend  upon  geotropism  to  guide  those  parts  into  the  suitable 
positions,  and  thus  to  realize  the  ultimate  shape  of  the  plant. 
And  this  is  the  case  no  matter  what  the  form  of  the  plant  may  be, 


Power  to  Adjust  Parts  to  Surroundings  251 

whether  a  symmetrical  cone  of  horizontally-spread  branches  ra- 
diating from  a  central  main  stem,  as  in  the  Firs  or  the  Spruces, 
or  a  great  urn  of  up-and-outcurved  branches,  as  in  the  Maples 
and  Elms,  or  in  any  of  the  intermediate  shapes;  and  the  reader 
should  learn  to  visualize  all  of  the  main  trunks  and  branches  as 
thus  developing  in  touch  with  gravitation  and  largely  under  its 
guidance.  This  applies,  however,  only  to  the  main  structures; 
the  smaller  branches  and  most  of  the  minor  parts  are  more  or 
less  controlled  by  other  kinds  of  stimuli  which  determine  the 
final  details  of  form;  and  this  is  especially  the  case  with  roots. 
The  fact  that  geotropism  is  thus  ever  tending  to  hold  the  plant 
to  a  certain  upright  symmetrical  form  explains  why  any  one- 
sided turning  in  response  to  other  stimuli,  is  of  limited  amount, 
and  why  the  plant  always  tends  to  recover  its  former  upright  and 
symmetrical  position  in  case  it  is  disturbed. 

Some  minor  tropisms. — These  include,— THERMOTROPISM,  a  turn- 
ing towards  warmth,  rather  rare:  TRAUMATROPISM,  the  turning 
of  roots  away  from  an  external  irritation  or  injury:  RHEOTROPISM, 
a  turning  against  a  water  current,  which,  however,  has  been 
shown  to  be  only  a  special  phase  of  thigmotropism :  ELECTRO- 
TROPISM,  a  certain  adjustment  to  mild  electric  currents;  and  some 
others  of  lesser  importance.  The  case  of  rheotropism,  by  the 
way,  illustrates  a  confusion  of  stimuli,  the  root  apparently  mistak- 
ing the  pressure  of  the  flowing  water  for  that  of  some  hard  ob- 
ject in  the  soil.  The  case  of  electrotropism,  involving  response 
to  an  influence  to  which  the  plant  is  never  subjected  in  nature 
and  to  which  it  cannot  have  become  adaptively  sensitive,  illus- 
trates the  same  thing,  or  else,  perhaps,  an  accidental  release  of  the 
motor-mechanism  after  the  manner  already  described  for  the 
Sensitive  Plant.  And  the  occasional  responses  found  in  plants 
to  other  stimuli  new  to  them  (e.  g.  to  X-rays,  radium  emana- 
tions), are  likewise  due  without  doubt  to  confusion  of  stimuli,  or 
accidental  release. 

Thus  far  we  have  considered  for  the  most  part  only  cases  in 


252  The  Living  Plant 

which  the  stimuli  act  from  a  single  direction,  and  therefore  evoke 
only  one-sided  responses.  But  some  of  the  very  same  stimuli 
may  act  in  a  diffused  or  all-around  manner,  becoming  impressed 
on  the  sensitive  protoplasm  of  the  plant  through  a  change  in 
intensity;  and  in  such  cases  the  responses  are  all-sided  or  sym- 
metrical. Thus  the  sleep  movements  of  leaves,  already  con- 
sidered, are  of  this  nature,  being  a  response  to  the  change  hi  in- 
tensity of  the  circumambient  light;  and  the  same  thing  occurs 
with  some  flowers,  which  close  at  night  or  in  very  dark  weather. 
Other  flowers,  e.  g  Tulips,  are  affected  in  like  manner  by  changes 
of  temperature,  opening  as  the  weather  grows  warmer,  and  clos- 
ing as  it  becomes  cooler;  and  some  evergreen  leaves,  notably  of 
Rhododendrons,  rise  and  fall  in  this  way  even  in  winter.  Such 
responses  are  distinguished  from  the  ordinary  sort  in  scientific 
terminology  by  the  termination,  nasty  (photonasty,  thermonasty, 
etc.) ;  and  we  may  note  by  the  way,  that  the  responses  due  to  a 
free-swimming  movement,  as  in  the  case  of  the  antherozoids  of 
Ferns  already  described,  are  distinguished  by  the  termination 
taxis  (chemotaxis,  phototaxis,  etc.). 

There  are,  furthermore,  several  other  types  of  responses  to 
stimuli,  some  of  them  vastly  important  in  connection  with  the 
growth  and  development  of  plants.  Thus,  it  has  been  claimed 
that  the  strains  set  up  by  the  swaying  of  stems  back  and  forth, 
whether  in  nature  by  winds,  or  in  the  laboratory  under  experi- 
ment, serve  as  stimuli  to  the  larger  development  of  strengthen- 
ing tissues  in  the  places  where  the  strains  are  most  felt,  thus  pro- 
ducing a  needed  enlargement  at  those  places.  It  is  perfectly 
clear  that  the  great  knees  which  rise  from  the  roots  of  the  Bald 
Cypress  of  the  Southern  Swamps  and  which  probably  are  aerating 
structures,  are  formed  in  response  to  the  presence  of  water,  for 
they  do  not  form  at  all  when  these  trees  grow  in  soil  that  is  well- 
drained.  Other  cases  are  known  where  the  thickness  of  cell-walls, 
the  arrangement  of  tissues,  the  sizes  of  parts,  and  other  structural 
features  are  regulated  by  responses  to  well-known  stimuli  from 


Power  to  Adjust  Parts  to  Surroundings  253 

the  environment.  Again,  the  climbing  roots  of  some  Ivies,  and 
the  sucking  roots  of  some  parasites,  grow  out  at  those  places 
where  the  stimulus  of  contact  is  felt,  and  therefore  exactly  at  the 
places  where  they  can  best  serve  their  uses;  and  the  places  of 
origin  of  even  ordinary  roots  are  largely  controlled  by  the  stimulus 
of  especially  abundant  moisture  or  minerals,  which  explains  why 
roots  branch  so  profusely  upon  entering  drains.  Then  there  are 
stimuli  which  start  particular  stages  of  growth.  Thus  it  is  a 
stimulus  given  by  some  phase  of  fertilization  which  starts  the 
formation  of  the  fruit  in  the  higher  plants.  The  advantage  is 
clear,  since  the  fruit  would  be  wasted,  and  its  formation  a  useless 
drain  on  the  plant,  if  no  fertile  seed  were  produced;  for  the  dis- 
persal of  the  seed  is  the  function  for  which  the  fruit  exists.  Stimuli 
can  also  serve  as  signals  to  produce  a  cessation  of  growth,  as  in 
the  case  of  the  leaves  of  the  water-plants  already  considered; 
and  there  are  plenty  of  other  cases  where  stimuli  regulate  growth 
and  development  in  various  ways,  the  further  consideration  of 
which  we  may  postpone  to  the  chapters  which  deal  with  those 
subjects,  where  also  we  may  consider  the  correlation  and  linking 
of  stimuli,  with  their  very  important  consequences. 

There  is  one  other  phase  of  responsiveness  to  stimuli  which  we 
must  consider  at  this  place.  It  is  a  familiar  fact  about  organisms 
that  they  have  a  certain  power  of  adjusting  themselves,  or  be- 
coming toned,  as  it  were,  so  as  to  work  their  best  under  the  pre- 
vailing conditions  to  which  they  are  exposed;  and  when  they  are 
thus  working  in  full  harmony  with  those  conditions  they  are  said 
to  be  in  tone.  We  have  a  familiar  illustration  thereof  in  our 
human  affairs  in  the  way  we  become  accustomed  to  certain  pe- 
culiarities of  food,  temperature,  fresh  air,  occupations,  etc.,  to 
such  a  degree  that  we  become  uneasy  when  exposed  to  any  others, 
and  hasten  back  with  relief  to  the  congenial  conditions.  Thus, 
most  of  us  work  our  best  at  about  70°  Fahrenheit  and  become 
very  uncomfortable  when  the  temperature  rises  to  above  90°, 
though  this  is  still  much  less  than  the  natural  heat  of  our  bodies. 


254  The  Living  Plant 

Moreover  this  condition  of  tone  is  more  or  less  alterable  under 
continuous  action  of  new  conditions,  and  such  tonic  adjustment 
to  new  conditions  is  commonly  called  acclimatization.  We  do  not 
yet  know  much  as  to  the  nature  of  the  process,  but  there  seems 
little  doubt  that  it  is  chemical  in  its  nature,  and  represents  a 
process  of  chemical  adjustment  to  the  external  conditions  acting 
as  stimuli.  An  important  phase  of  the  same  process  is  found  in 
the  formation  in  the  animal  body  of  those  special  chemical  sub- 
stances called  collectively  " antibodies,"  which  neutralize  chemi- 
cally the  injurious  substances  formed  in  disease.  Probably  the 
acquisition  of  tone  and  acclimatization  are  fundamentally  similar 
in  principle,  consisting  in  chemical  alterations  in  the  protoplasm 
of  such  character  that  substances  or  features  less  efficient  under 
the  prevailing  conditions  are  replaced  by  others  more  efficient. 
At  least  such  seems  to  be  the  principle,  though  as  to  the  details, 
they  are  still  with  the  future. 

As  one  views  the  various  adjustive  structures  produced  in 
response  to  external  stimuli  (such  as  the  knees  of  the  Bald  Cypress 
just  mentioned,  the  thicker  epidermis  of  plants  in  dry  places, 
and  so  forth),  one  cannot  but  ask  how  these  may  be  distinguished 
from  adaptive  structures  produced  in  the  course  of  evolution; 
and  whether,  after  all,  the  two  may  not  be  fundamentally  the 
same  thing.  As  to  the  first  point,  one  cannot  distinguish  adjustive 
from  adaptive  structures  by  any  evidence  except  the  test  of 
heredity,  for  adjustive  structures  are  produced  anew  in  each 
generation  only  in  response  to  certain  stimuli  and  are  absent 
when  the  stimuli  are  lacking,  while  adaptive  structures  are  pro- 
duced regularly  every  generation  quite  regardless  of  the  presence 
or  absence  of  the  given  stimulus.  The  only  thing  that  is  hereditary 
in  irritable  adjustments  is  the  capacity  to  make  them.  We  have 
an  analogy  in  the  different  methods  whereby  republics  and 
monarchies  are  provided  with  rulers,  for  while  the  president  of  a 
republic  is  often  indistinguishable  in  mode  of  life  and  other 
characteristics  from  a  monarch,  and  may  even  surpass  one  in 


Power  to  Adjust  Parts  to  Surroundings  255 

power,  he  is  chosen  quite  anew  at  regular  intervals  in  adjustment 
to  the  popular  demands  of  the  moment,  only  the  method  of  elect- 
ing him  being  permanent,  or,  so  to  speak  hereditary;  while  the 
monarch  holds  his  office  by  heredity  quite  regardless  of  the 
fluctuations  of  politics.  As  to  our  second  question,  whether  in 
the  last  analysis,  the  two  may  not  be  fundamentally  the  same, 
adaptive  structures  being  only  permanently-fixed  irritable  ad- 
justments, the  view  is  attractive  but  as  yet  unproven,  as  we  shall 
further  consider  in  the  chapter  on  Evolution. 

There  remains  one  other  important  matter  to  mention  in  con- 
nection with  stimuli.  The  response  to  a  stimulus,  while  highly 
efficient,  is  blindly  invariable,  and  not  alterable  for  particular 
conditions.  For  example,  if  a  wind-blown  seed  of  an  ordinary 
plant  were  to  lodge  in  a  cleft  of  an  overhanging  ledge,  it  would 
be  an  advantage  for  this  plant  to  be  able  to  reverse  the  usual 
positions  of  roots  and  stem;  yet  we  know  it  would  send  its  stem 
up,  though  only  to  die  in  the  earth,  and  its  root  down,  only  to 
perish  in  the  air.  In  this  invariability  of  particular  responses, 
and  in  many  respects  besides,  these  irritable  responses  of  plants 
agree  with  the  reflex  actions  familiar  in  animals;  and  it  is  now 
very  clear  that  they  are  essentially  the  same.  Furthermore,  if 
two  or  more  stimuli  act  upon  the  same  part  of  the  plant  at  the 
same  time,  the  result  is  simply  the  product  of  the  effort  of  the 
part  to  respond  to  them  all.  There  is  no  sign  of  an  attempt  on 
the  side  of  the  plant  to  correlate  these  stimuli,  so  to  speak,  and 
to  respond  in  a  manner  which  would  be  best  in  the  face  of  this 
particular  combination.  In  this  respect  animals  have  gone  far 
ahead  of  plants,  for  they  have  acquired  that  last-mentioned 
power.  Herein  we  have  the  chief  feature  which  distinguishes 
the  higher  animals  from  the  higher  plants,  and  also,  I  believe, 
the  origin  of  consciousness.  Thus,  out  of  one  and  the  same 
origin,  plants  have  developed  irritability,  while  animals  have 
developed  reflex  action,  consciousness,  and  ultimately  reason. 


CHAPTER  X 

THE  VARIOUS  WAYS  IN  WHICH  PLANTS  RESIST  THE 
HOSTILE  FORCES  AROUND  THEM 

Protection 

[Y  the  methods  considered  in  the  preceding  chapters, 
plants  provide  most  effectively  for  their  nutritive 
needs,  and  also  for  advantageous  adjustment  to  the 
external  conditions  affecting  the  same.  But  they  have 
not  thereby  solved  the  whole  problem  of  daily  existence,  for  they 
still  have  to  reckon  with  the  presence  of  a  great  many  hostile 
external  conditions.  Thus  the  winds,  which  in  moderation  do 
no  damage  to  plants  and  even  may  work  them  some  benefit, 
occasionally  swell  to  great  tempests  possessing  a  power  well- 
nigh  too  great  for  resistance.  Again,  water,  which  is  indispensa- 
ble to  plants  in  considerable  quantity,  becomes  sometimes, 
through  drought,  quite  dangerously  scant,  or  through  floods 
quite  as  dangerously  plenty;  while  various  parts  and  places  of 
the  earth, — deserts  on  the  one  hand  and  swamps  on  the  other — 
though  perfectly  habitable  by  plants  in  all  other  respects,  remain 
permanently  hi  one  or  the  other  of  these  undesirable  conditions. 
Further,  light,  which  is  likewise  essential  to  plants,  is  in  some 
times  and  places  too  weak  for  efficiency,  and  in  others  so  intense 
that  unprotected  protoplasm  can  by  no  means  endure  it.  And 
again,  the  food  supply  manufactured  by  plants,  while  ordinarily 
ample  for  both  themselves  and  their  hereditary  dependents  the 
animals,  is  in  some  parts  indispensable  to  the  continuance  of  the 
plants'  activity,  so  that  its  destruction  by  animals  would  consti- 
tute a  serious  menace.  Finally,  while  endowed  with  indefinitely 

256 


How  Plants  Resist  Hostile  Forces  Around  Them  257 

great  powers  of  reproduction  and  growth,  plants  live  in  a  world 
already  quite  filled,  and  are  therefore  exposed  to  a  competitive 
struggle  with  one  another,  of  which  natural  selection  is  the  re- 
morseless arbiter,  and  a  survival  of  the  fittest  the  inevitable 
outcome.  In  a  word,  plants  live  in  a  world  that  is  generally 
friendly,  but  sometimes  is  hostile  even  to  a  mortal  degree. 
Against  the  hostile  features  of  the  environment  they  have  had 
to  develop  protective  adaptations,  some  of  which  are  extremely 
conspicuous  and  play  a  large  part  in  the  determination  of  the 
habits  and  aspects  of  plants.  These  protective  adaptations,  of 
course,  must  co-exist  and  compromise  with  those  physiological 
adaptations  in  leaf,  stem,  and  root,  which  we  have  already  con- 
sidered. The  identification,  separation,  and  definition .  of  the 
structures  and  features  of  plants  which  are  protective  is  the  task 
that  now  lies  before  us. 

To  begin  with,  the  protoplasm  of  plants  is  physically  weak, 
but  secures  an  efficient  first  line  of  defense  by  the  most  obvious 
of  all  methods,  viz.,  through  encasing  each  one  of  the  soft-bodied 
cells  in  a  separate  coating  of  armor, — the  cell-wall.  As  the 
reader  will  recall,  from  the  description  in  the  chapter  on  Proto- 
plasm, the  plant  skeleton  is  constructed  from  the  united  wall- 
mass  of  the  cells;  and  it  thus  combines  both  support  and  seclu- 
sion for  the  protoplasm  in  its  cavities,  very  much  as  the  walls  of 
our  many-storied  houses  do  for  us.  Such  a  combination  of  skele- 
ton and  protecting  wall  is  permitted  only  by  the  sedentary  habits 
of  plants,  and  stands  in  very  great  contrast  with  animals,  whose 
locomotive  habit  requires  a  jointed  skeleton,  moved  by  masses 
of  contractile,  and  therefore  naked  (muscular)  cells. 

Turning  now  in  detail  to  the  various  hostile  influences  against 
which  plants  need  protective  adaptations,  the  most  obvious  is 
that  of  the  winds,  which,  however,  become  a  danger  only  as  they 
rise  into  gales.  Then,  as  all  will  agree  who  have  seen  a  great 
tree  tossed  in  the  grasp  of  a  tempest,  protection  is  found  in  the 
slenderness  and  elasticity  of  the  branches,  which  yield  in  great 


258  The  Living  Plant 

curves  that  permit  the  smaller  to  stream  with  the  wind  in  the 
lee  of  the  larger,  where  they  can  tug  at  their  anchorage  in  safety. 
Doubtless  in  a  windless  world  the  plant  skeleton  would  be  rigid 
and  brittle,  probably  to  such  a  degree  that  an  ordinary  one  of 
our  storms  would  shatter  it  to  fragments,  much  as  at  times  they 
do  now  with  the  ice  of  a  silver  thaw.  As  to  older  stems,  we  have 
learned  already  how  it  is  with  them;  their  hollow-column  princi- 
ple of  construction  holds  them  up  against  great  lateral  strains. 
Furthermore,  a  good  many  kinds  of  stems  exhibit  a  special 
strengthening  arrangement  at  the  place  of  maximum  weakness, 
which  lies  at  the  contact  of  stem  and  root,  where  the  leverage 
exerted  by  wind  on  the  top  is  most  felt.  Thus,  some  kinds  of 
plants,  like  the  Corn,  develop  prop  roots  that  extend  from  the  stem 
above  ground  diagonally  down  to  the  earth,  while  many  tall 
trees  possess  buttress-like  thickenings  between  the  stem  and  the 
principal  roots,  as  appears  very  well  in  some  of  our  Elm  trees, 
and  especially  in  some  of  the  tropical  giants,  where  they  attain  a 
good  many  feet  of  height  and  breadth,  though  only  a  few  inches 
of  thickness.  As  to  leaves,  whose  broad  faces  would  present 
much  exposure  to  wind,  their  slender-elastic  petioles  permit  them 
to  yield,  and  to  swing  like  so  many  weather  vanes,  presenting 
only  edges  to  the  blast,  while  they  can  also  sway  accommoda- 
tingly to  every  irregular  gust.  In  this  adaptation,  indeed,  we 
find  one  of  the  principal  functions  of  the  petiole,  as  follows  from 
a  discovery  made  by  one  of  my  own  students, — who  found  that 
the  petioles  from  the  exposed  part  of  a  tree  average  longer  than 
those  from  more  sheltered  situations,  although  the  leaves  are 
smaller  in  the  former  locations  than  the  latter. 

But  it  is  not  alone  on  the  individual  tree  that  the  sizes  of  leaves 
are  inversely  proportional  to  the  degree  of  their  exposure  to 
winds,  for  it  is  true  in  general  of  plants  as  a  whole.  Do  not  the 
largest  leaves  that  are  known  to  the  reader  grow  in  the  shelter 
of  undergrowth?  And  if  at  first  sight  it  appears  that  the  gigantic 
fronds  of  Palms  and  Tree  Ferns  contradict  this  view,  a  second 


How  Plants  Resist  Hostile  Forces  Around  Them    259 

thought  is  enough  to  confirm  it;  for,  although  morphologic- 
ally single  leaves,  they  are  cleft  to  a  great  many  small  leaflets, 
each  of  which  acts  physiologically  as  a  single  leaf.  This  division, 
or  "compounding"  (as  it  is  called  scientifically),  of  leaves  in  such 
plants  appears  clearly  to  constitute  a  protective  adaptation 
against  the  tearing  action  of  winds;  and  I  believe  the  same  factor 
is  the  principal  one  in  determining  the  compounding  of  leaves  in 
general,  though  sometimes  the  compound  condition,  as  in  our 
undergrowth  Ferns,  means  rather  a  persistence  of  an  ancestral 
condition  than  anything  of  immediate  importance.  Nor  is  one's 
natural  thought  at  this  point,  that  the  sizes  of  leaves  are  de- 
pendent on  their  thickness,  correct.  The  thickness  of  leaves  is 
determined  by  the  depth  to  which  sunlight  can  penetrate  green 
tissues  without  losing  all  of  its  photosynthetic  power;  and  hence 
it  is  approximately  the  same  in  all  leaves  exposed  to  the  sun  in 
the  same  climate,  with  a  trend  towards  more  thickness  in  extra- 
bright  places,  and  thinness  in  shade.  Undoubtedly  the  whole 
tendency  of  wind  action  is  to  produce  an  adaptive  lessening  in 
size,  which  is  directly  antagonistic  to  the  tendency  of  photo- 
synthesis to  produce  a  larger  spread  of  surface;  and  the  resultant 
between  the  action  of  these  two  factors,  modified  it  is  true  by 
certain  other  minor  influences,  makes  leaves  the  sizes  they  are. 
This  explains  why  our  common  deciduous  trees  of  similar  habit, 
our  Oaks,  Elms,  Maples,  and  Chestnuts,  possess  leaves  of  much 
the  same  size,  or  at  least  of  the  same  order  of  magnitude.  That 
size  represents  the  equilibrium  between  the  contesting  photo- 
synthetic  and  wind  factors  acting  on  leaves  of  standard  thick- 
ness growing  in  similar  situations. 

Another  kind  of  strain  to  which  plants  are  exposed  is  the 
weight  of  the  winter's  snow  and  ice.  This  danger  is  greater,  of 
course,  for  evergreen  than  deciduous  trees,  but  against  it  the 
conical  shape  characteristic  of  evergreens  provides  a  manifest 
protection.  This  follows  from  the  fact  that  only  the  ends  of  the 
branches  are  exposed  to  the  falling  load,  while  their  slender  forms 


260  The  Living  Plant 

and  horizontal  positions  permit  them  to  yield  greatly  without 
damage,  and  thereby  even  to  shed  their  burdens  (figures  14,  15). 
No  doubt  the  protective  adaptation  involved  in  the  conical  shape 
has  operated  along  with  the  photosynthetic  considerations 
earlier  mentioned  (page  56)  to  fix  this  form  for  evergreen  trees, 
which  in  general  are  commonest  in  the  snowiest  regions;  while, 
correlatively,  the  danger  involved  in  the  accumulation  of  snow 
upon  the  leaves  borne  by  upwardly  springing  branches,  like  those 
of  most  of  our  deciduous  trees,  is  doubtless  one  factor  in  making 
such  trees  drop  their  leaves  in  the  winter. 

This  mention  of  the  shapes  of  trees  makes  this  a  suitable  place 
to  consider  their  modes  of  resistance  to  certain  other  strains. 
The  stems  of  trees  have  not  only  to  carry  great  masses  of  foliage 
high  up  in  the  air,  but  also  to  support  it  out  laterally  for  con- 
siderable distances,  and  all  in  opposition  to  a  heavy  downward 
strain  from  gravitation.  In  some  trees,  conspicuously  those  of 
the  cone-shaped  evergreen  type  (figures  14,  15),  the  branches 
spread  horizontally  from  a  central  upright  trunk;  but  this  ar- 
rangement, however  advantageous  from  other  points  of  view, 
is  mechanically  the  worst  for  resistance  to  gravitational  strains, 
and  is  only  possible  with  comparatively  slender  branches  and 
special  methods  of  strengthening  the  same.  Thus,  bracket-like 
swellings  often  occur  in  the  angles  between  such  branches  and 
stems,  while  extra  material  is  commonly  placed  all  along  the 
under  side  of  the  branch,  making  it  excentric  in  cross  section.  In 
such  cases  the  extra  material  acts  much  like  a  long  stiff  spring 
bent  upward  just  enough  to  counterbalance  the  weight  of  the 
branch,  wnose  horizontal  position  is  maintained  by  the  counter- 
action of  the  two  forces,  as  is  shown  quite  conclusively  by  the 
very  great  bending  of  such  branches  when  spring  and  weight  are 
allowed  to  act  together  by  the  inversion  of  the  tree  (figure  90). 
But  a  cone-shape  of  trees  is  uncommon  in  comparison  with  that 
in  which  great  branches,  often  well-nigh  as  large  as  the  trunk,  rise 
up  therefrom  at  sharp  angles,  swing  gradually  outward  to  near 


How  Plants  Resist  Hostile  Forces  Around  Them    261 


the  young  parts,  and  then  curve  vertically  upwards  again  to 
bear  the  new  leaves, — the  whole  stem  melting  away,  as  it  were, 
to  a  spray  of  such  branches.  This  is  the  form  prevailing  in  most 
of  our  deciduous  trees,  as  the  reader  can  see  for  himself  by 
examining  the  tracery  of 
Oaks,  Maples,  Elms,  or 
Chestnuts  when  projected 
against  the  winter  sky. 
Such  a  sigmoid  form  of 
the  branches  affords  them 
the  best  possible  anchor- 
age in  the  trunk  with  the  FJG  90._Tracings  from  photographs  of  the  same 

minimum     Of     leverage     On        Balsam  Fir,  in  the  natural  position  and  inverted, 
.  .  illustrating  a  point  explained  in  the  text. 

their  heaviest  parts,  while 

providing  enough  spread  and  a  vertical  tip  for  support  of  the 
foliage.  If,  now,  we  apply  this  sigmoid  mechanical  modification 
to  the  theoretical  form  of  our  photosynthetic  tree  represented  in 
figure  7,  we  obtain  the  form  illustrated  herewith  (figure  91). 
This  theoretical  form,  modified  by  some  minor  and  largely  acci- 
dental circumstances,  is  very  nearly  realized  in  the  noble  Oak 
shown  in  figure  8,  and  by  many  of  our  common  deciduous  trees. 
The  chief  difference  consists  only  in  this,  that  whereas  the 
theoretical  tree  is  hemispherical,  the  actual  kinds  are  often 
ovoid,  cylindrical,  or  top-shaped, — in  obvious  adaptation,  as  I 
think,  to  a  diminution  of  the  excessive  gravitational  leverage 
that  accompanies  too  extensive  a  spread. 

We  pass  now  to  a  second  of  the  greater  environmental  in- 
fluences hostile  to  plants,  namely  excessive  light.  The  reader 
does  not  need  to  be  told  that  light,  and  in  large  quantity,  is  in- 
dispensable to  plants  for  their  photosynthetic  work;  but  it  is  an 
important  physical  fact  that  the  amount  they  can  thus  use  has  a 
limit,  above  which  any  increase  is  not  only  useless  but  positively 
harmful.  And  that  limit  is  often  surpassed  in  the  open  sunlight 
of  summer.  However,  not  all  of  the  mani-colored  rays  that  make 


262 


The  Living  Plant 


up  the  white  light  are  thus  injurious,  but  only  the  blue-violet, 
and  then  only  when  received  in  great  force;  for  these  very  same 
rays,  like  some  of  the  red,  are  the  ones  that  are  useful  in  photo- 
synthesis. They  produce  their  bad  effects,  as  it  seems,  through 

their  peculiar  power  of  promoting 
chemical  changes,  whereby  they  in- 
duce in  the  complicated  living 
protoplasm  illegitimate  reactions, 
as  it  were,  which  interrupt  the  or- 
derly series  of  chemical  processes 
in  which  the  very  life  of  the 
protoplasm  consists.  However, 
whether  this  be  the  correct  ex- 
Cation  or  not,  it  is  nevertheless 

a  fact  that 


mechanical  support  of  the  weight  of    because  of   its  blue   rays,   is  always 
the  foliage.  ...  ,  .    .  mi  • 

injurious  to  living  protoplasm.   1  his 

is  the  reason  why  bright  light  is  fatal  to  disease  germs,  or  Bacteria, 
and  explains  the  basis  of  the  hygienic  value  of  sunlight  in  the 
home;  while  blue  light  is  used  with  success  for  the  very  same  reason 
in  the  cure  of  some  diseases  of  the  skin.  Now  because  the  red 
rays  of  the  sunlight  are  not  only  harmless  but  also  useful,  even  in 
fullest  intensity,  while  the  blue  rays  are  harmful  only  when  in- 
tense, but  otherwise  useful,  the  problem  of  adaptive  protection 
against  too  intense  light  resolves  itself  into  one  of  tempering  the 
blue  rays  without  affecting  the  others.  This  can  be  perfectly 
accomplished  through  use  of  a  screen  which  permits  red  rays  to 
pass  while  checking  the  blue,  and  such  a  screen  is  of  necessity 
red.  It  is  upon  precisely  this  principle  that  photographers  use  a 
ruby  glass  screen  in  developing  their  plates,  for  this  color  cuts 
off  the  blue  rays,  which  are  those  that  took  the  picture  originally 
and  therefore  would  spoil  it  in  development,  while  admitting  the 
red  rays  which  are  not  only  harmless  to  the  plate  but  useful  in 
showing  the  photographer  what  he  is  doing;  —  only  the  photog- 


How  Plants  Resist  Hostile  Forces  Around  Them    263 

rapher  needs  a  total  exclusion  of  blue  rays  and  therefore  a  screen 
of  much  deeper  color  than  the  plant  requires  for  only  a  partial 
exclusion  of  those  rays.  Such  is  most  likely  the  adaptive  signifi- 
cance of  that  charming  red  blush  which  mantles  the  face  of  the 
fresh  vegetation  of  spring,  for,  without  some  such  protection, 
the  young  leaves  and  stems  that  push  out  of  the  buds  before 
the  formation  of  the  chlorophyll,  which  constitutes  later  a  suffi- 
cient though  incidental  protection,  would  expose  then-  unshielded 
protoplas/n  to  the  full  force  of  the  bright  light  then  prevailing. 
And  there  are  some  students  who  find  a  similar  function  in  the 
redness  of  leaves  in  the  autumn,  believing  that  it  shields  the 
protoplasm  after  the  chlorophyll  has  faded  away;  though  here,  as 
I  believe  and  have  argued  in  the  second  chapter,  there  is  little 
warrant  in  the  evidence.  Certain  it  is  that  there  are  cases,  e.  g., 
the  red  under  sides  of  leaves  of  some  tropical  undergrowth 
plants,  where  the  explanation  must  be  totally  different.  But 
the  light-screen  function  explains  very  well  the  reddish  or  brown- 
ish colors  of  spores  which  must  float  long-time  in  the  air  exposed 
to  the  brightest  of  light,  and  perhaps  it  explains  also  the  red 
color  assumed  by  roots  and  underground  stems  when  these  be- 
come exposed  to  the  light,  though  here  the  color  may  represent 
simply  a  chemical  incident. 

A  second  method  of  light  protection  may  consist  in  those  hairy 
or  woolly  coatings,  or  even  in  the  waxy  or  resinous  layers,  which 
overspread  a  good  many  plants  of  open  bright  places,  resulting 
in  a  distinctive  aspect  of  grayness  found  especially  often  in  plants 
of  the  deserts.  Such  covers  must  act  to  reflect  and  refract  the 
light,  without,  of  course,  any  distinction  of  rays,  to  an  extent  suf- 
ficient to  weaken  very  greatly  its  power  to  penetrate  the  tissues. 

The  third  of  the  methods  of  light  protection,  bound  up,  how- 
ever, with  protection  against  excessive  transpiration  soon  to  be 
noted,  is  more  important.  It  consists  in  the  assumption  by  the 
green  tissues  of  a  vertical  position,  whereby  they  present  only  a 
thin  edge,  or  at  least  a  low  angle  of  incidence,  to  the  mid-day 


264  The  Living  Plant 

brightness  of  the  sun,  with  the  full  exposure  to  its  less  intense 
action  at  morning  and  evening.  Such  a  vertical  position  of  the 
green  surface  is  common  in  plants  of  open  bright  places, — in 
some,  notably  certain  clover-like  kinds,  as  a  temporary  and 
irritably-adjustable  position  of  the  leaflets  (figure  78),  but  in 
others  as  a  permanently  vertical  arrangement  of  the  leaves.  In 
the  most  perfect  of  the  latter  cases,  all  the  leaf-blades  present 
their  faces  to  the  east  and  the  west,  thus  bringing  their  edges 
north  and  south;  and  such  is  the  real  meaning,  and  the  reason 
for  the  name,  of  the  Compass  Plants,  of  which  the  most  perfect 
and  famous  example  occurs  on  our  own  western  prairies.  In 
some  kinds,  instead  of  the  leaf-blade  it  is  the  petiole  which  is 
flattened  and  set  vertically,  the  blade  being  suppressed,  as  in 
most  of  the  Australian  Acacias  (figure  21).  In  others,  advantage 
is  taken  of  the  naturally  vertical  position  of  the  stem,  the  function 
of  foliage  being  transferred  thereto  from  the  leaves  which  are 
simultaneously  reduced  or  abandoned.  This  is  the  case  with  the 
Cactuses  and  innumerable  other  plants  of  the  deserts,  which 
sometimes  acquire  additional  vertical  green  surface  by  the  de- 
velopment of  longitudinal  ribs.  The  readiness  with  which  the 
green  tissue  can  be  developed  in  one  part  of  the  plant  as  well  as 
another  helps,  by  the  way,  to  explain  some  of  the  curious  mor- 
phological overturnings  represented  by  plants  like  the  Butcher's 
Broom  (figure  23),  or,  still  better,  the  familiar  Smilax  of  the 
florists,  in  which  the  apparent  leaves  are  in  reality  branches, 
while  the  actual  leaves  are  no  more  than  tiny  scales  just  beneath 
them.  It  is  easy  to  understand  that  if  plants  of  the  desert  have 
once  transferred  their  chlorophyll  to  their  stems,  simultaneously 
suppressing  or  abandoning  their  leaves,  and  then  a  change  of 
climate,  or  migration  to  a  moister  region,  should  require  a  larger 
spread  of  green  surface,  this  would  more  easily  and  naturally  be 
secured  through  a  further  flattening  of  the  stems  or  branches  than 
through  a  restoration  of  the  lost  leaves;  and  with  tune  such 
branches  would  become  more  and  more  leaf-like  even  to  the 


How  Plants  Resist  Hostile  Forces  Around  Them    265 

extreme  degree  represented  by  the  Smilax.  This  very  over- 
turning does  actually  occur  in  the  Cactus  family,  in  which, 
happily,  all  of  the  steps  without  exception  are  represented  by  still 
living  forms.  It  is  the  relics,  indeed,  of  such  devious  windings  in 
the  past  history  of  plants  which  give  us  our  principal  morpho- 
logical puzzles. 

This  consideration  of  light  naturally  suggests  the  question  as 
to  heat.  This,  likewise,  is  indispensable  to  plants,  since  it  supplies 
a  condition  requisite  for  some  chemical  reactions  and  physical 
movements,  notably  diffusion.  Heat  also,  like  light,  is  more 
and  more  useful  up  to  a  certain  intensity  (about  that  of  blood 
heat  in  ourselves),  beyond  which  any  increase  is  not  only  without 
benefit,  but  soon  becomes  an  injury.  Thus,  plants  in  the  fields  in 
summer  by  no  means  thrive  better  the  hotter  it  gets.  It  is  doubt- 
ful, however,  whether  the  natural  heat  of  the  sun  ever  attains 
an  intensity  dangerous  to  plants,  and  even  if  it  does,  the  same 
structural  adaptations,  especially  refractive  coverings  and  a 
vertical  position  of  green  tissues,  protective  against  light,  would 
be  equally  effective  against  heat.  And  there  is  perhaps  yet 
another  method  of  protection  against  both,  but  especially  heat, 
namely  transpiration,  which  dissipates  through  evaporation  the 
too  intense  energy  of  heat  and  light  thrown  into  the  leaf  by  the 
sunlight,  as  we  have  noted  already  (page  209).  There  is,  how- 
ever, one  place  on  the  earth's  surface,  and  that  is  in  hot  springs, 
where  low  kinds  of  plants  belonging  to  the  Algae  can  grow  at  a 
much  higher  temperature  than  the  sun  ever  produces, — even  a 
degree  too  hot  for  the  hand  to  endure  (up  to  81°  Centigrade  or 
192°  Fahrenheit).  In  these  Alga?  no  structural  adaptations  to 
protection  occur  (unless  a  certain  slimy  coating  be  such,  though 
this  is  hard  to  believe),  but  the  living  protoplasm  has  apparently 
become  acclimatized  to  the  high  temperature,  probably  by  the 
elimination  of  all  chemical  constituents  affected  thereby  and  the 
substitution  of  others  whose  reactions  are  under  full  control  at 
such  temperatures. 


266  The  Living  Plant 

Thus  much  for  heat;  at  the  other  end  of  the  thermometric 
scale  more  abundant  and  better  marked  adaptations  are  known, 
for  the  natural  temperatures  of  the  earth  do  fall  plenty  low 
enough  to  prove  fatal  to  working  plant  protoplasm.  The  first  of 
the  methods  of  protection  against  cold  consists  in  the  elimination 
of  water,  for  while  moist  and  working  protoplasm  is  killed  near 
the  freezing-point,  the  dry  substance  can  endure  temperatures  of 
more  than  200°  Centigrade  (over  400°  Fahrenheit)  below  zero 
without  injury, — and,  by  the  way,  in  this  condition,  can  also 
endure  heat  even  above  the  boiling  point  of  water.  This  power 
of  resistance  of  dry  protoplasm  against  cold  and  heat  is  doubt- 
less due  to  the  fact  that  the  injury  resulting  therefrom  is  of  a 
chemical  nature,  and  the  chemical  changes  in  living  protoplasm 
proceed  only  in  solution,  and  solution  requires  water. 

The  protection  against  cold  afforded  by  dryness  explains  how 
seeds,  which  become  very  dry,  can  withstand  such  low  tempera- 
tures. Winter  buds,  however,  and  the  other  living  tissues  of 
plants  become  only  partially  dry  in  winter,  and  consequently 
are  only  partially  protected  by  this  method;  the  remainder  of 
their  safety  is  probably  secured  by  the  slight  amount  of  heat 
released  in  respiration,  which  continues  all  winter,  and  which 
is  effectively  conserved  by  the  non-conducting  wrappings 
provided  in  the  air-holding  bark,  and  the  woolly  coatings  of  buds. 

From  these  reasonably  certain  adaptations  we  turn  to  some 
others  of  rather  a  doubtful  sort.  The  leaflets  of  many  kinds  of 
plants,  and  the  flowers  of  some  others,  close  together  or  "sleep" 
at  night;  and  Darwin,  who  studied  these  movements  most 
closely,  thought  they  must  form  a  protection  against  too  great 
cooling  at  night.  This  has  been  doubted  of  late,  and  apparently 
with  reason,  but  nobody  has  given  as  yet  any  more  probable 
explanation.  Again,  the  red  color  of  the  spring  vegetation  has 
been  thought  to  provide  a  kind  of  mitigation  of  the  effects  of  cold 
weather  then  frequent,  in  that  by  a  certain  power  it  possesses  of 
converting  light  into  heat,  it  warms  up  the  parts  in  the  bright 


How  Plants  Resist  Hostile  Forces  Around  Them    267 

but  cool  days  of  early  spring,  when  all  the  warmth  procurable  by 
the  plant  is  desirable  for  hastening  the  development  of  the 
various  parts.  This  explanation  has  been  applied  in  particular 
to  the  red  stigmas  and  styles  of  wind-pollinated  flowers  which 
ripen  before  the  appearance  of  the  leaves  in  the  spring,  the  extra 
warmth  thus  acquired  being  supposed  to  promote  the  growth  of 
the  pollen-tube  and  hence  to  hasten  the  fertilization.  But  here 
we  are  nearing  mere  guesswork,  and  must  not  accept  such  sug- 
gestions as  explanation,  but  simply  as  interesting  hypotheses 
deserving  of  determination  through  the  test  of  experiment. 

We  come  now  to  the  most  deadly  of  all  the  dangers  to  which 
plants  are  exposed, — and  that  is  dryness  As  the  reader  will 
readily  recall,  water  is  not  only  the  principal  constituent  of  the 
bodily  structure  of  plants  and  indispensable  in  their  daily  nutri- 
tion, but  is  also  evaporating  or  transpiring, — constantly,  co- 
piously, and  unavoidably, — from  all  of  their  younger  aerial  parts. 
Therefore  plants  need  a  constant  and  uniform  water  supply,  but 
in  fact  rarely  get  it,  for  the  most  of  the  kinds,  including  all  of 
those  most  familiar  to  us,  live  under  conditions  of  extreme 
variability  not  only  as  to  the  quantity  available  for  absorption 
by  the  roots,  but  also,  and  especially,  as  to  the  quantity  forcibly 
transpired  from  their  tissues, — these  conditions,  indeed,  being 
linked  with  the  most  variable  of  all  things,  the  weather.  Against 
such  fluctuations  ordinary  plants  secure  a  tolerable  protection, 
on  the  one  hand  through  their  power  of  absorbing  even  the 
hygroscopic  water  of  the  soil  through  their  copious  root  hairs, 
and,  on  the  other,  by  their  complete  waterproof  epidermis,  the 
few  necessary  openings  in  which  are  automatically  regulated, 
albeit  somewhat  clumsily,  in  adjustment  to  the  prevailing  condi- 
tions. But  in  places  where  water  is  permanently  scant,  as  it  is 
extremely  in  deserts,  these  simple  arrangements  are  insufficient 
and  must  be  supplemented  by  special  protective  adaptations; 
and  these  take  three  different  forms,  under  which  heads  we  shall 
consider  them, — (a),  increased  efficiency  of  the  absorbing  system, 


268  The  Living  Plant 

(b),  development  of  water-storing  tissues  and  organs,  (c),  ar- 
rangements that  minimize  loss  by  transpiration. 

The  absorbing  system  of  typical  plants,  as  the  reader  now 
knows  very  well,  consists  principally  in  the  innumerable  root 
hairs,  which  draw  water  osmotically  from  a  wide  area  all  around 
them.  Plants  that  live  in  dry  places  usually  exhibit,  either  as  an 
individual  adjustment  or  a  structural  adaptation,  a  marked 
intensification  of  one  or  more  of  the  features  involved  in  this 
absorption; — that  is,  the  number  of  young  roots  is  larger,  the 
hairs  are  more  profuse,  the  osmotic  solutions  are  stronger,  or  the 
total  range  of  the  root  system  through  the  soil  is  greater.  The 
increased  profusion  of  hairs  in  drier  situations  is  manifest  when 
young  roots  are  grown  in  damp  air,  where  they  make  a  far 
greater  display  than  ever  they  do  in  the  soil;  while  the  much 
wider  range  and  greater  freedom  of  branching  attained  by  root 
systems  in  plants  that  grow  in  dry  places,  helps  to  explain  why  it 
is  that  the  plants  of  the  deserts  are  spaced  so  widely  apart,  with 
large  open  areas  between  them.  The  presence  of  stronger  osmotic 
solutions  inside  the  absorbing  root  hairs  is  distinctive  not  only  of 
some  desert  plants,  but  also  of  others  which  grow  in  a  different 
situation  where  water  is  hard  of  absorption  even  though  present 
in  plenty, — namely,  in  salt  marshes,  where  the  water  itself  is  a 
markedly  osmotic  solution  of  appreciable  strength.  As  was 
shown  in  the  chapter  on  Absorption,  osmotic  absorption  by  roots 
is  dependent  on  a  superiority  in  strength  of  the  inner  over  the 
outer  solution,  and  is  the  slower  and  harder  the  more  nearly  the 
two  approach  the  same  concentration.  It  is  this  difficulty  of 
osmotic  absorption  from  salt  water  which  explains  why  large 
vegetation,  while  crowding  as  close  as  it  can  to  fresh-water 
streams  and  lakes,  keeps  away  from  the  corresponding  situations 
along  the  margin  of  the  sea. 

The  storage  of  water  is  the  second  of  the  methods  protective 
in  plants  against  dryness.  All  living  cells  of  all  plants,  indeed, 
possess  plentiful  stores  of  water  in  their  sap-cavities,  which 


How  Plants  Resist  Hostile  Forces  Around  Them    269 

explains  no  doubt  the  reason  for  the  prevalence  of  the  large  cell- 
cavity  in  the  construction  of  plant  cells.  But  many  of  the  plants 
of  dry  places  develop  great  numbers  of  specially-large  cells  ob- 
viously adapted  to  water  storage  in  particular,  and  the  presence 
of  such  cells  makes  the  parts  that  contain  them  swollen,  rounded, 
soft-textured  and  translucent.  This  is  the  origin  of  the  type  of 
plant-structure  commonly  described  as  succulent,  and  distinctive 
of  many  Cactuses,  Euphorbias,  Mesembryanthemums,  House- 
leeks,  and  others,  all  of  which  grow  either  in  deserts,  or  in  other 
places,  such  as  the  clefts  of  rocky  hills,  where  water  is  scanty  for 
long  times  together.  This  storage  of  water  is  naturally  com- 
bined in  a  great  many  cases  with  the  storage  of  food,  in  which 
case  the  parts  display  a  firmer  texture  and  whiter  aspect  in 
section,  as  illustrated  for  example  by  the  Century  Plants.  And 
the  examples  above  given  show  that  the  storage  organs  can  be 
leaves,  as  well  as  stems,  while  roots  are  frequently  used  for  the 
same  purpose. 

The  minimization  of  transpiration  is  the  third  and  most  im- 
portant of  the  protective  adaptations  against  dryness.  We  have 
noted  already  the  method  by  which  ordinary  plants  are  protected 
against  drought,  viz.,  the  possession  of  a  waterproof  epidermis 
whose  only  openings,  the  stomata,  are  protectively  guarded. 
Now  there  is  apparently  no  limit  to  the  thickness  and  perfection 
of  waterproofing  that  can  be  given  by  plants  to  their  epidermis; 
and  if  it  were  possible  for  them  to  exist  without  the  stomata, 
then  plants  in  dry  places  could  wrap  themselves  up  in  a  way  to 
conserve  their  indispensable  water  without  limit.  But  as  the 
reader  well  knows,  green  plants  in  order  to  live  must  have  food, 
which  is  made  by  photosynthesis,  which  requires  a  supply  of 
carbon  dioxide,  which  must  be  drawn  from  the  atmosphere  out- 
side. Thus  is  necessitated  the  existence  of  the  stomata,  which 
must  be  open  for  a  time  and  extent  directly  proportional  to  the 
food  to  be  made;  and  this  means  that  water  will  escape,  or  tran- 
spire, incidentally  but  inevitably,  through  those  openings  to  an 


2  yo  The  Living  Plant 

amount  proportional  to  the  food  manufactured.  This  inevitable 
linking  of  transpiration  with  photosynthesis  is  one  of  the  most 
fundamental  of  all  facts  in  the  economy  of  green  plants.  Some 
plants  of  dry  places,  especially  the  deserts,  have  solved  the 
adaptive  problem  thus  presented  by  condensing  all  of  their 
photosynthetic  work  into  the  brief  moist  season  (for  all  deserts 
where  plants  can  grow  at  all  do  possess  such  a  season),  spending 
the  remainder  of  the  year  in  a  resting  and  dried  state,  com- 
parable with  that  assumed  by  our  plants  over  winter.  But 
others,  illustrated  by  the  Cactuses  conspicuously,  continue  their 
activity  all  through  the  season,  in  reliance  upon  a  copious  store 
of  water  and  sundry  devices  for  rigid  economy  of  the  same. 

This  matter  of  economy  in  transpiration  is  so  important  and 
interesting  that  we  must  give  it  a  little  further  consideration. 
The  first  and  most  obvious  method  thereof  consists  in  the  re- 
duction of  total  green  surface,  which  of  course  is  carried  to  a 
degree  sufficient  to  keep  the  unavoidable  loss  within  the  limit  of 
safety.  This  is  the  reason  why  the  plants  of  dry  places,  and 
especially  of  the  deserts,  are  comparatively  small,  why  they  are 
so  commonly  compacted  in  form,  and  why  they  so  often  are 
leafless, — the  very  object  of  the  existence  of  the  leaf,  as  the 
reader  will  recall,  being  that  of  spreading  more  surface.  A 
second  method  consists  in  the  provision  of  an  especially  efficient 
epidermis,  preventive  of  transpiration  except  through  the  regula- 
ble stomata;  and  so  thick  and  strong  does  it  become  in  some 
plants  that  it  is  hard  to  cut  and  impossible  to  compress  with  the 
hands,  and  actually  resembles  a  coating  of  horn  spread  all  over 
the  plant,  as  some  of  the  Cactuses  illustrate.  But  a  third  and 
most  interesting  method  of  transpiration  economy  consists  in 
certain  arrangements  which  hinder  somewhat  the  transpiration 
without  interfering  with  the  gas  diffusion.  This  to  some  degree 
is  accomplished  by  a  vertical  position  of  the  green  tissues,  for 
while  the  diffusion  of  carbon  dioxide  through  the  stomata  takes 
place  with  equal  facility  in  any  position  of  the  tissues,  the  tran- 


How  Plants  Resist  Hostile  Forces  Around  Them    271 

spiration  is  much  less  from  vertical  surfaces,  since  the  force  of 
the  sun  which  supplies  the  transpiration  energy  is  obviously 
much  less  powerful  upon  vertical  than  horizontal  surfaces. 
Doubtless,  by  the  way,  this  factor  is  much  more  potent  than 
those  of  protection  against  light  and  heat  in  determining  the 
prevailingly  vertical  position  of  green  tissues,  whether  of  stems 
or  of  leaves,  in  plants  of  dry  and  desert  places.  And  this  con- 
clusion is  strongly  confirmed  by  the  fact  that  salt  marsh  plants, 
which  need  protection  against  much  transpiration  though  hardly 
at  all  against  light  and  heat,  especially  in  northern  regions,  show 
a  notable  tendency  to  a  vertical  position  of  leaves  and  other  green 
tissues.  But  the  very  same  end  is  also  attained  in  a  different 
way  by  the  provision,  outside  of  the  stomata,  of  chambers  in 
which  the  escaping  vapor  is  held  for  a  time,  thus  checking  tran- 
spiration a  little,  somewhat  as  a  damp  atmosphere  would  do, 
while  the  inward  diffusion  of  carbon  dioxide  is  not  appreciably 
affected.  In  some  plants  these  chambers  consist  of  deep  pits  in 
the  thick  epidermis  with  guard  cells  lying  at  the  bottom;  in 
others  the  same  effect  is  produced  by  coatings  of  hairs  or  scales; 
while  in  still  others  the  leaves  are  inrolled  to  tubes  into  which  the 
stomata  all  open.  The  same  result  follows,  as  well,  from  the 
dense  crowding  together  of  leaves,  such  as  desert  plants  show  not 
infrequently  (figure  12,  center).  And  many  other  arrangements, 
notably  hardness  of  tissues,  and  the  presence  of  gelatinous  sub- 
stances, both  contributing  to  water  conservation,  have  been 
described. 

Thus  much  for  dangers  from  dryness;  the  other  extreme, — 
too  much  water, — likewise  constitutes  at  times  a  danger  to 
plants,  rarely,  however,  in  any  direct  manner,  but  indirectly 
through  prevention  of  the  access  of  air  supply.  But  this  matter 
has  already  been  considered  along  with  respiration  and  aeration, 
where  the  various  protective  adaptations  (air  passages,  aerating 
structures,  utilization  of  the  dissolved  air  of  the  water), — have 
been  sufficiently  described.  A  very  different  kind  of  protective 


272  The  Living  Plant 

adaptation  against  damage  by  water  has  been^claimed  by  those 
who  believe  that  transpiration  is  not  simply  an  unavoidable 
evil,  but  a  process  of  value  in  itself.  The  presence  of  water  on 
leaves,  derived  from  dew  or  the  rain,  must  check  transpiration, 
partly  by  blocking  the  stomata,  and  partly  by  the  creation  of  a 
moist  atmosphere  around  the  leaves  during  its  evaporation;  and 
any  arrangements  tending  to  prevent  the  wetting,  or  facilitate 
the  drying,  of  leaves  would  thus  be  protective.  Such  arrange- 
ments do,  apparently,  exist  in  those  waxy  or  other  unwettable 
coatings  which  enable  leaves  to  shed  water  in  small  drops  as  one 
can  see  readily  in  the  Garden  Nasturtium,  the  Pond  Lilies,  and 
a  great  many  others,  some  of  which  show  a  silvery  film  of  air  all 
over  the  leaf  when  plunged  into  a  vessel  of  water.  The  same 
result  is  claimed  to  follow  in  a  different  way  in  those  very  many 
leaves  of  tropical  rainy  regions  which  taper  off  to  a  long  slender 
tip  ending  in  a  very  small  point ;  these  tips  collect,  as  it  were,  the 
water-drops  as  they  slip  down  the  hanging  leaf,  and  guide  them  to 
the  point  whence  their  own  weight  makes  them  drip  to  the 
ground,  leaving  the  surface  well  drained,  and  ready  the  sooner 
to  begin  transpiration. 

There  is,  however,  one  way  in  which  water  can  damage  plant- 
structures  directly,  and  it  actually  happens  with  grains  of  pollen 
when  these  are  touched  by  the  rain.  The  functions  and  mode  of 
growth  of  these  grains,  which  will  be  fully  described  in  the  follow- 
ing chapter,  is  such  that  their  walls  are  necessarily  thin  and  their 
contents  osmotically  attractive  to  water,  whence  it  happens  that 
if  they  become  touched  thereby,  they  absorb  it,  swell  up,  and 
burst,  as  can  be  seen  very  clearly  when  the  water  is  added  to 
grains  under  the  microscope.  The  pollen,  therefore,  needs  pro- 
tection from  rain,  whereto  a  good  many  adaptations  have  been 
found,  as  can  be  considered,  however,  more  appropriately  along 
with  the  Flower  in  the  chapter  devoted  to  that  subject.  It  has 
also  been  claimed  that  the  surface-cells  of  the  simple  and  soft- 
bodied  plants  of  fresh  water  are  subject  to  a  similar  osmotic 


How  Plants  Resist  Hostile  Forces  Around  Them    273 

absorption,  especially  in  very  warm  weather;  and  this  might,  if 
too  sudden,  produce  damaging  distension  and  perhaps  rupture 
of  the  walls.  But  these  plants  are  commonly  covered  by  a  thin 
coating  of  jelly,  which  is  known  to  greatly  impede  the  rapidity 
of  water-passage,  thus  allowing  enough  time  for  an  equalization 
of  pressures  through  the  stem.  This  is  very  likely  the  adaptational 
significance  of  the  jelly,  or  slime,  of  water  plants  generally. 

But  the  forces  of  the  air,  the  earth  and  the  waters  are  not  the 
only  ones  hostile  to  plants,  for  among  their  worst  enemies  are 
other  plants,  and  animals.  As  to  plant  enemies,  the  most 
deadly  are  the  parasites,  the  Bacteria,  Molds,  Mildews,  Blights, 
Rots,  Rusts,  Smuts,  and  other  Fungi  which  often  destroy  their 
host  plants  entirely.  Yet  few,  if  any,  positive  adaptations  have 
been  found  in  plants  protective  against  these  parasites,  although 
some  of  the  oils  and  resins  occasionally  found  in  leaves  do  appear 
to  afford  a  moderate  protection  against  Fungi  as  well  as  against 
animals.  Practically  all  of  these  plants  reproduce  by  spores 
which  are  blown  about  by  the  wind;  and  when  these  fall  upon 
suitable  plants  they  germinate  into  slender  threads.  These, 
for  the  most  part,  have  no  power  to  penetrate  the  epidermis, 
which  is  thus  someway  protective  against  them;  but  they  enter 
the  open  stomata  and  thus  reach  the  soft  food-filled  cells  of  the 
leaf,  which  they  proceed  to  devour.  Thus  the  necessity  for  the 
existence  of  stomata  involves  another  danger  besides  that  of 
excessive  transpiration,  and  in  this  case  one  against  which  plants 
seem  well-nigh  helpless.  However,  plants  differ  immensely, — 
not  only  different  species  but  even  different  individuals  of  the 
same  species, — in  their  susceptibility  to  injury  by  parasitic 
Fungi,  and  there  is  very  good  reason  to  believe  that  the  differ- 
ences have  a  chemical  basis,  some  kinds  possessing  a  chemical 
constitution  hostile  to  the  growth  of  the  parasite  while  others 
do  not.  These  differences  offer  a  basis  for  the  attempts  now 
being  made  in  many  experiment  stations  to  combat  plant 
diseases  by  breeding  immune  varieties, — the  less  susceptible 


274  The  Living  Plant 

individuals  being  preserved  for  breeding  in  each  generation. 
And  perhaps  it  will  yet  be  found  that  plants  can  be  rendered 
immune  by  acclimatization,  so  to  speak,  to  their  diseases,  as 
animals  can  be  to  theirs.  But  of  these  matters  we  know  little 
as  yet. 

We  come  finally  to  the  last  of  the  environmental  factors  hostile 
to  plants,  and  that  is  the  depredations  of  animals.  These,  in- 
deed, cannot  be  otherwise  than  constant  and  great,  since  in  the 
long  run  every  jot  of  the  food  that  is  eaten  by  animals  has  to  be 
ravaged  from  plants.  The  general  defense  of  plants,  however,  is 
passive  and  indirect,  consisting  chiefly  in  a  reliance  upon  their 
own  superabundant  powers  of  growth,  regeneration,  and  repro- 
duction, in  which  features  they  surpass  animals  many  fold.  So 
great  are  these  powers,  indeed,  that  plants  are  enabled  to  produce 
organic  material  in  vast  excess  of  their  own  needs,  upon  which 
fact  depends  the  very  possibility  of  the  existence  of  animal  life. 
And  it  may  be  true  that  the  use  of  this  surplus  by  animals  is  not 
only  of  no  damage  to  plants,  but  may  even  be  useful  in  removing 
superfluous  material  and  making  room  for  a  more  active  evolu- 
tion of  that  which  remains.  In  any  case  it  is  a  generous  payment 
for  the  various  services  which  animals  render  to  plants. 

Although  plants  thus  appear  to  possess  few  adaptations  against 
animal  attacks,  especially  in  their  vegetative  parts,  there  appear 
to  be  notable  exceptions.  Thus,  a  good  many  herbs  develop 
various  substances  in  their  stems  or  their  leaves, — bitter  oils, 
turpentiny  resins,  acrid  glucosides,  astringent  tannins,  alkaloids, 
or  even  needle-pointed  irritating  crystals, — which  render  those 
plants  distasteful  to  herbivorous  animals,  including  all  kinds 
from  the  greatest  of  beasts  down  to  slugs  and  innumerable  in- 
sects. Everybody  has  noticed  the  clumps  of  such  plants  left 
standing  quite  isolated  in  pastures  by  cattle  which  browse  the 
grass  well-nigh  to  the  roots  all  around  them.  In  these  cases, 
however,  the  protection  is  perhaps  incidental  rather  than  adapt  a- 
tional,  and  may  be  defensive  against  Fungi  rather  than  animals; 


How  Plants  Resist  Hostile  Forces  Around  Them    275 

but  there  are  other  instances  where  the  actual  adaptation  seems 
reasonably  clear.  Thus,  in  the  desert  the  conditions  of  life  are  so 
hard  that  plants  can  scarcely  produce  any  surplus  above  their 
own  needs,  while  the  very  precious  store  of  water  laid  up  in  their 
stems,  and  essential  to  their  own  safety  throughout  the  dry  sea- 
son, is  particularly  tempting  to  large  animals.  In  such  plants, 
accordingly,  we  find  the  best  development  of  features  that  ap- 
pear to  be  most  protective  against  animals, — either  distasteful 
secretions,  which  are  especially  common  and  virulent  in  plants 
of  the  deserts,  or  else  a  horrid  armature  of  thick-set  and  dan- 
gerous prickles  and  spines  through  which  animals  can  penetrate 
but  painfully  if  at  all.  Furthermore,  a  few  desert  plants  are 
known  which  resemble  so  closely  the  background  against  which 
they  grow, — either  the  rough  gray  surface  of  the  soil  (as  in  the 
case  of  the  half -buried  flat-topped  "Living  Rock"  Cactus  of  the 
American  Southwest),  or  else  the  drab  pebbles  in  the  beds  of  dry 
water-courses  (as  in  a  number  of  plants  described  from  the 
peculiar  flora  of  South  Africa), — that  it  seems  as  if  such  plants 
must  surely  escape  notice  by  animals,  and  secure  a  protection, 
by  this  form  of  mimicry,  though  here  again  it  may  be  true  that 
the  result  is  incidental  rather  than  adaptational.  But  while  a 
protective  mimicry  seems  reasonable  in  plants  of  this  kind  and 
habit,  the  same  can  hardly  be  said  of  those  cases  in  the  flora  of 
ordinary  climates  where  some  kinds  have  been  claimed  to  secure 
protection  through  their  resemblance  to  Nettles,  or  Poison  Ivy, 
or  other  plants  actually  noxious.  Indeed,  so  far  as  concerns  a 
protective  function  for  the  poison  of  plants  like  Poison  Ivy,  there 
is  a  difficulty  in  the  fact  that  the  repelling  effect  is  not  felt  until 
long  after  the  plant  has  been  injured.  I  think  we  do  not  yet  know 
the  meaning  of  the  poisonous  quality  of  plants. 

An  injury  done  to  the  vegetative  parts  of  plants  does  not  ex- 
tend to  other  parts,  and  is  easily  replaced;  but  damage  to  the 
machinery  of  growth  and  reproduction  is  serious,  in  correlation 
with  which  fact  we  find  in  those  parts  a  good  many  apparently 


276  The  Living  Plant 

protective  adaptations.  Thus,  the  preservation  of  the  store  of 
food  laid  up  for  starting  the  new  vegetation  in  the  Spring  is 
obviously  of  vital  importance.  We  find  in  fact  that  the  sugar 
and  starch  stored  up  by  perennial  or  biennial  plants  is  placed 
underground  in  bulbs,  tubers,  or  rootstocks,  where  it  is  well  out 
of  the  sight  and  reach  of  large  animals,  especially  when  the 
ground  is  frozen  in  winter;  while  in  woody  perennials,  where  the 
food  must  remain  largely  above  ground,  it  is  scattered  thinly 
throughout  a  large  area  of  tough  woody  tissue.  As  to  seeds,  they 
are  often  protected  by  hard  coats  or  extra  shells,  impervious,  for 
the  most  part,  not  only  to  gnawing  teeth  but  also  to  the  digestive 
juices  of  animals,  though  in  the  case  of  large  nuts  the  teeth  of  the 
squirrels  have  won  a  trifle  of  the  same  advantage  over  the  hard- 
ness of  the  shells  that  the  gunmakers  have  won  over  the  armor 
makers  among  our  own  civilized  selves,  and  doubtless  after  much 
the  same  kind  of  long  evolutionary  struggle  between  the  two. 
Adaptations  to  protection  of  the  food  supply  in  these  nuts  while 
still  growing  and  before  the  hard  seed  coats  are  formed  have 
been  found  in  such  spines  as  the  Chestnut  and  Horse  Chestnut 
display:  in  the  bitter  taste  of  some  pods:  and  in  their  green  color 
which  has  been  taken  for  protective  coloration,  though  it  is  also 
readily  interpretable  as  simply  the  usual  utilization  of  all  avail- 
able surfaces  for  the  spread  of  more  chlorophyll.  The  greenness 
of  edible  fruits  prior  to  their  ripeness  has  also  been  interpreted  as 
protective  until  the  tune  when  they  turn  red  or  other  color  and 
aid  in  dissemination  of  the  seeds,  as  we  shall  consider  at  length  in 
the  fifteenth  chapter.  Flowers,  likewise,  exhibit  some  remark- 
able adaptations  to  protection  against  animals,  though  in  a 
different  way,  and  combined  with  some  more  remarkable  adapta- 
tions for  attracting  them,  though  this  is  a  subject  which  can  be 
considered  more  conveniently  in  our  chapter  on  the  Flower.  As 
to  protective  adaptations  of  the  growth  machinery,  which  is 
principally  the  buds,  there  appear  to  be  several.  Not  only  are 
the  buds  important  as  the  originators  of  new  growth,  but  con- 


How  Plants  Resist  Hostile  Forces  Around  Them    277 

sisting  as  they  do  of  a  rich  store  of  succulent  food,  they  cannot 
but  prove  attractive  to  smaller  animals.  Thus,  a  good  many 
kinds,  as  in  the  Grasses,  place  their  buds  underground,  whence 
they  send  up  their  stems  and  leaves  to  the  light.  In  buds  that 
must  grow  in  the  air,  every  soft  protoplasmic  growing  point  is 
deeply  buried  by  the  leaves  it  is  forming,  for  these  at  first  lie 
tightly  against  it,  and  only  later  open  out  to  the  light.  Their 
green  color,  moreover,  must  afford  an  appreciable  measure  of 
protective  coloration,  which  is  doubtless  the  chief  explanation 
of  the  prevalent  greenness  of  the  calyx  of  flowers.  In  some 
desert  plants,  where  true  leaves  are  wanting,  the  buds  are  sunken 
deep  in  a  hollow  formed  by  the  older  tissue,  and  often  are  further 
protected,  as  in  the  Cactuses,  by  a  perfect  cheveux  de  fris  of 
tough  and  interlocking  spines.  The  growing  points  of  roots  are 
well  protected  by  their  position  underground,  while  the  cambium 
cylinder,  likewise  richly  stored  with  the  most  nutritious  of  food, 
is  deeply  enwrapped  by  the  tough  fibrous  bark,  which  often 
contains  in  addition  a  great  deal  of  tannin, — a  substance  strongly 
distasteful  to  most  gnawing  animals. 

In  viewing  this  list  of  adaptations,  one  is  constantly  reminded 
of  the  fact  that  they  never  are  perfect  in  operation,  since  animals 
do  successfully  attack  plants  against  every  one  of  these  protec- 
tions. Like  so  many  others  of  the  adaptations  of  plants,  they 
are  real  and  are  useful,  though  clumsy.  However,  they  do  obvi- 
ously afford  a  considerable  measure  of  protection,  enough,  as  it 
seems,  to  permit  plants  to  hold  their  own  in  the  struggle;  and  in 
a  world  that  is  full  this  is  all  that  they  need. 


CHAPTER  XI 

THE  WAYS  IN  WHICH  PLANTS  PERPETUATE  THEIR  KINDS, 
AND  MULTIPLY  THEMSELVES  IN  NUMBER 

Reproduction 


?  all  the  facts  about  life,  no  one  is  more  fundamental 
or  familiar  than  this, — that  individuals  inevitably  die. 
Obviously  they  must  be  replaced  if  the  race  is  to  con- 
tinue, and  this  replacement  is  the  office  of  reproduction, 
which  we  must  now  proceed  to  consider.  Our  study  of  the  subject 
will  have  all  the  more  interest  for  the  reason  that,  like  several 
others  of  the  physiological  processes,  reproduction  is  substantially 
identical  in  meaning  and  method  in  animals  and  plants,  differing 
only  in  some  external  features  connected  with  their  differences 
in  habit.  Therefore  any  knowledge  acquired  in  one  kingdom  can 
be  transferred  to  the  other;  and  one  may  learn  from  plants  the 
essential  nature  of  reproductive  processes  in  animals,  including 
mankind. 

The  central  fact  of  reproduction  is  the  formation  of  new  in- 
dividuals capable  of  growing  into  kinds  closely  like  those  which 
produced  them.  Associated  therewith,  however,  is  the  further 
fact  that  usually  the  formation  of  a  new  individual  requires  the 
cooperation,  through  the  act  of  fertilization,  of  two  parent  in- 
dividuals of  different  sexes;  and  so  prominent  is  this  feature, 
especially  among  the  higher  animals,  that  most  people  consider 
it  an  indispensable  feature  of  reproduction.  This  idea,  however, 
is  not  correct,  and  the  two  things, — formation  of  new  individuals 
and  sexual  union, — though  so  often  associated,  are  quite  inde- 
pendent in  their  nature,  as  is  shown  by  the  fact  that  purely 
non-sexual,  or  asexual,  reproduction  exists  abundantly,  not  only 

278 


How  Plants  Perpetuate  Their  Kinds  279 

among  plants,  but  among  the  simpler  animals  as  well.  We  may 
best  consider  this  asexual  type  before  proceeding  to  the  more 
familiar  sexual  kind. 

Asexual  reproduction  is  effected  through  the  separation  of  a 
portion  of  plant  structure  capable  of  growing  into  a  new  plant 
without  any  preliminary  fertilization  or  other  influence  of  sex. 
In  the  higher  and  more  familiar  plants 
it  is  rather  rare.  Almost  anybody  can 
recall  the  dark,  ovoid  bodies  which  are 
borne  by  Lilies  in  the  axils  of  their  leaves 
(figure  92),  and  which  easily  separate 
and  produce  new  plants.  These,  though 
seemingly  seeds,  are  really  a  kind  of 
bulblet  specialized  for  this  sort  of  veae- 

FIG.  92.— A  portion  of  stem,  with 

tative     multiplication;    and    equivalent     leaves,  of  a  Lily,  showing  the 

i      j-  fit  ?  .1  axillary  bulblets  mentioned  in 

bodies  are  found  also  on  a  few  other  the  text.  (Copied  from  Gray's 
kinds  of  plants.  Again,  the  runners,  structural  Botany.) 
suckers,  stolons,  and  other  similar  structures  sent  over  or  under 
the  ground  by  Strawberries,  Blackberries,  Grasses,  and  many 
wide-ranging  weeds,  produce  vegetative  shoots  at  their  tips,  and 
thus  propagate  vegetatively  while  securing  a  kind  of  dissemina- 
tion, as  we  shall  note  more  fully  in  our  chapter  on  the  last- 
mentioned  subject.  It  is,  however,  a  fact  of  great  interest,  and 
likewise  of  much  practical  consequence,  that  although  most  of  the 
higher  plants  have  lost  their  power  of  propagating  themselves 
vegetatively,  they  can  yet  be  made  artificially  to  reproduce  in  that 
way.  Thus,  the  propagation  of  plants  by  slips  or  cuttings  of  any 
sort  is  artificial  vegetative  reproduction.  Great  numbers  of  plants 
will  strike  root  of  themselves  and  grow  when  slips  thereof  are 
placed  in  the  earth;  others  which  will  make  no  roots  in  this  way 
can  be  made  to  do  so  by  various  devices  of  gardeners;  while  still 
others  which  cannot  be  made  to  strike  roots  at  all  can  yet  be 
fitted  with  a  set,  so  to  speak,  by  the  operation  of  grafting,  as  the 
reader  will  learn  more  fully  in  our  later  chapter  on  Growth. 


280  The  Living  Plant 

Asexual  reproduction  in  the  lower  and  simpler  plants  takes 
place  in  two  principal  ways.  In  the  tiniest  plants,  which  com- 
prise the  Bacteria,  or  Germs,  and  some  of  the  simplest  Seaweeds, 
the  entire  plant  body  consists  of  no  more  than 
a  single  cell,  which  in  reproduction  splits  di- 
rectly across  the  middle;  the  two  parts  then 
grow  promptly  to  full  size,  only  to  split  again, 
and  so  on  without  limit  (figure  93),— a  method 
called  fission,  or  division.  Nothing  could  be 
simpler,  which  explains  the  extreme  rapidity  of 

FIG.  93.— Stages  in  the 

division  of   a   one-  multiplication  in  the  forms  that  possess  it.    And 

C^s  highly  Sagm"-  Jt  is  interesting  to  observe  that  this  is  exactly 

ftu'aplrtecel(copied  the  method  whereby  single  cells  reproduce  in 

from    the    Chicago  even  the  highest  of  plants.     A  second  method 

Textbook). 

of  asexual  reproduction  in  the  simpler  plants 
consists  in  the  formation  of  asexual  spores,  which,  under  great 
diversity  of  habit  and  form,  exhibit  these  features  in  common, 
— that  they  are  single  cells  separated  off  from  the  parent  plant, 
and  capable  of  growth  directly  each  to  a  new  plant.  In  some 
Seaweeds  they  are  provided  with  swimming  appliances  whereby 
they  can  move  through  the  water  in  a  manner  so  suggestive  of 
animals  that  they  are  known  scientifically  as  zoospores  (figure 
94),  though  in  other  Seaweeds 
they  are  drifted  passively  about 
by  the  water-currents.  Non- 
motile  spores  occur  in  many 
land  plants; — are  formed  in  pro- 
fusion in  the  gills  Of  Mushrooms,  FIG.  94. — Forms  of  free-swimming  repro- 
thp  ransillps  of  Mos^PS  thp  ductive  cells,  of  which  the  two  on  the  left 
>es>  T  are  asexual  zoospores,  while  the  two  on 

brown    SpOtS     (or    SOri),    On    the        the^right  are  sexual  cells,  later  to  be  de- 

under   sides   of   the   fronds   of 

Ferns,  and  in  the  black  stalked  heads  that  develop  on  various 
Molds  (figure  95),  whence  they  are  wafted  on  the  wings  of  the 
wind  to  the  uttermost  parts  of  the  earth.  Oft-times  these  asexual 


How  Plants  Perpetuate  Their  Kinds  281 

spores  are  provided  with  coats  of  such  thickness  and  hardness  as 
to  make  them  immune  against  every  natural  condition  of  dry- 
ness  and  heat  for  weeks,  months  or  years  together;  which  fact,  in 
conjunction  with  their  power  of  floating  with  the  dust  in  the  air, 
explains  their  ubiquitous  penetration  into 
all  kinds  of  strange  places.  It  is  thus 
that  Bacteria  and  Yeasts,  for  example, 
are  enabled  to  spread  so  widely  as  they 
do. 

Such  is   asexual   reproduction,   which 
never  involves  fertilization,  and  has  no 
relation   whatsoever   to   sex.     In   sharp 
contrast  stands  sexual  reproduction,  in-  FIG.  95.—  The  spore  case  of  a 
volving  the  cooperation,  through  fertilize 


tion,  of  two  parents  which  are  usually,  t(Jrr°™y  Bre™dh)~C°pied  PiC" 
though  not  always,  of  different  sexes. 

We  may  best  begin  our  study  of  fertilization  with  the  more  famil- 
iar plants,  where  it  occurs  in  the  flower,  which  is  a  structure 
specially  adapted  thereto.  In  a  typical  flower,  as  everyone 
knows,  the  outer  green  protective  calyx  and  the  inner  colored 
showy  corolla  together  enclose  the  stamens  and  the  pistils  (fig- 
ure 96).  The  stamen  consists  of  a  slender  stalk  crowned  by  a 
chamber,  the  anther,  containing  a  fine  yellow  dust,  the  grains  of 
pollen,  inside  of  which  develop  the  male  cells  of  the  plant.  The 
pistil  consists  of  a  rounded  chamber,  the  ovary,  extending  upwards 
into  a  lengthened  stalk,  the  style,  ending  in  a  roughened  swelling, 
the  stigma,  and  containing  one  or  more  ovules  in  which  develop 
the  female  cells  of  the  plant.  If,  further,  a  critical  examination  is 
made  of  a  typical  ovule,  by  aid  of  longitudinal  sections  and 
microscope  (figure  97),  it  is  found  to  enclose  inside  of  some  coats 
a  definite  cavity,  the  embryo  sac,  within  which  in  turn  is  consider- 
able protoplasm  and  several  cells,  including  a  larger  one  close  to 
the  end  where  an  opening  (the  micropyle}  is  left  through  the  coats. 
This  larger  cell  is  the  female  generative  cell,  the  exact  equivalent 


282  The  Living  Plant 

of  the  egg  in  animals,  and  hence  called  the  egg-cell  Thus  much 
for  the  female  reproductive  apparatus;  turning  now  to  the  male, 
we  find  it  in  the  pollen  grain,  which,  examined  microscopically, 
is  found  to  consist  of  at  least  two  cells  (figure  98),  of  which  one 

gives  rise  to  the  male,  or 
sperm,  cells,  presently  to  be 
further  described.  Such  is 
the  floral  structure,  and  such 
the  appearance  and  locations 
of  the  sexual  cells,  when  fully 
ripe  and  ready  for  fertiliza- 
tion. 

The  process  of  fertilization 
itself  can  be  followed  in  de- 
tail by  aid  of  the  microscope, 

FIG.  90.—  Interior  view  of  a  typical  flower  (of  .  .  "" 

Peony),  showing  the  four  distinctive  parts,—    and  IS  shown  in  essentials  by 

our  generalized  drawing  (fig- 


stamens,  and  the  two  pistils,  which  in  this    ure  gg\       fj^  g^  g.          -g        ,_ 
case   show  typical    ovaries,   but   very    short 

styles  and  small  stigmas.    (Copied  from  Imation,  or  the  transfer  of  the 

Strasburger's  Textbook). 

pollen  grain  from  the  anther 

to  the  stigma,  and  since  the  pollen  is  usually  brought  from  a  sepa- 
rate plant,  the  process  is  far  more  elaborate  than  one  would 
imagine,  and  one,  withal,  which  involves  so  many  striking  and 
interesting  features  that  we  must  treat  the  subject  in  a  chapter 
by  itself;  and  that  chapter  follows.  When  the  pollen  grain 
reaches  the  stigma,  to  which  it  is  held  by  a  certain  roughness 
aided  by  a  sugary  stickiness,  it  immediately  begins  to  send 
out  a  slender  tube.  This  tube,  which  carries  in  its  tip  two 
nuclei  that  represent  the  essential  parts  of  two  male  cells,  grows 
down  into  the  tissues,  through  which  it  dissolves  its  way 
by  aid  of  enzymes  secreted  by  the  tip,  the  dissolved  substance 
being  absorbed  for  food;  and  thus  the  tube  literally  digests  its  way 
down  through  the  tissues  of  stigma  and  style  to  the  cavity  of  the 
ovary.  Here  it  passes  out  from  the  solid  tissue,  and,  guided  as  it 


How  Plants  Perpetuate  Their  Kinds  283 

seems  by  some  chemical  vapor  exuded  from  the  micropylar  open- 
ing of  the  ovule,  grows  straight  thereto  and  enters,  pursuing  its 
way  until  its  tip  comes  to  lie  flat  against  the  egg-cell.  Then  one 
of  the  male  nuclei  moves  out  of  the  tube  into  the  egg-cell  (fig- 
ure 100),  and  across  it  to  the 
nucleus  thereof;  then  the  two 
nuclei  touch,  flatten  a  bit 
against  one  another,  and 
finally  fuse  and  intermingle 
completely.  Thus  the  egg- 
cell  comes  to  possess  a  nucleus 
made  up  from  the  two  nuclei 
derived  from  the  two  parents, 
male  and  female;  and  this  is 
the  central  and  most  essen- 
tial feature  of  fertilization. 
The  fertilized  egg-cell  is  now 
ready  to  grow  into  an  em- 
bryo, which,  with  certain  ac- 
cessory parts,  forms  the  seed, 
and  later  grows  to  an  adult 

new  plant.  FIG.  97.— A  typical  ovule  (of  Narcissus),  seen  in 

I'U  A  'U    J  optical  longitudinal  section,  highly  magnified. 

have  described  somewhat      The  parts  may  be  identified  from  the  text, 

fllllv  this  nroppss  of  iWtiliyn-  the  most  important  being  the  egg-cell,  which 

miy  l  is  the  larger  of  the   three   cells  lying  in  the 

tion   as  it   OCCUrS  in   an  Ordi-  upper  end  of  the  embryo  sac.    (Copied  from 

.     .  a  wall-chart  by  Dodel-Port). 

nary  plant  because  it  is  typi- 
cal in  principle  of  all  fertilization  through  the  plant  and  animal 
kingdoms.  The  machinery  varies  immensely  of  course  in  detail. 
In  some  kinds  of  plants  the  sperm  cell  is  not  carried  by  a  growing 
tube,  but,  guided  by  certain  attractive  chemicals  exuded  by  the 
female  parts,  swims  of  itself  in  water  to  the  egg-cell,  as  is  the  way 
in  Ferns,  Mosses  and  many  Seaweeds.  In  most  animals,  how- 
ever, the  male  cells  (called  spermatozoids)  are  brought  by  suitable 
organs  to  the  near  vicinity  of  the  egg-cells,  to  which  they  finally 


284  The  Living  Plant 

swim  of  themselves.  But  in  all  cases  the  principle  is  the  same;  by 
suitable  structures  and  accessory  adjustments  and  adaptations, 
the  male  cell  is  brought  into  contact  with  an  egg- 
cell,  to  which  it  passes  over  its  nucleus,  with  more 
or  less  cytoplasm;  the  two  nuclei  then  fuse,  and 
thus  is  formed  a  fertilized  egg-cell  from  which  the 
new  individual  develops. 

In  following  this  process  it  becomes  evident  that 
the  object  of  all  the  elaborate  machinery  of  fertili- 
FIG.  98.—  A  typi-  zation  is  to  secure  the  union  of  the  male  and  female 

cal  pollen  grain  ,    .     .  ,  .        ,  ,.  i_  •    i_    • 

(of  Tradescan-  nuclei,  for  that  is  the  one  feature  which  is  com- 
ticai  ScTion  a°nd  Pletely  constant  throughout.  This  of  course  raises 
highly  magni-  the  question  as  to  what  the  nucleus  actually  is. 

fied,  showing,  on 

the  left,  the  cell  As  our  chapter  on  Protoplasm  showed,  every  nu- 

which    produces       ,  •  •  *.  n     i 

the    male,    or  cleus   contains   a   certain   peculiar  matter  called 


(Copied  Cf  rm  chromatin,  which,  ordinarily  scattered  throughout 
strasburger's  the  nucleus,  collects  itself  at  times  into  a  definite 
number  of  rod-like  structures  called  chromosomes 
(figure  101).  The  evidence  seems  to  show  beyond  question, 
though  the  method  thereof  is  in  doubt,  that  these  chromosomes 
embody  within  themselves  the  characteristics  of  the  parent 
plants,  (or,  constitute  the  working  plans  or  patterns  thereof, 
so  to  speak),  and  in  such  manner  as  to  exert  control  over  the 
development  of  offspring,  and  ensure  that  new  individuals  shall 
grow  up  in  resemblance  to  those  that  produced  them.  Now 
in  fertilization  each  nucleus  contributes  its  chromosomes,  so  that 
the  nucleus  of  the  fertilized  egg-cell  contains  a  double  number 
derived  equally  from  both  parents  (figure  100).  The  significance 
of  this  fact,  however,  becomes  apparent  only  as  we  follow  the 
behavior  of  the  chromosomes  during  the  development  of  the 
fertilized  egg-cell  into  an  adult  plant;  for  in  such  development, 
the  egg-cell  first  divides  across  into  two,  and  then  its  parts  into 
two,  and  so  on  until  the  whole  plant  is  completely  grown.  Now 
in  the  first  division  each  one  of  the  chromosomes,  both  those 


How  Plants  Perpetuate  Their  Kinds  285 

derived  from  the  male  and  those  derived  from  the  female  parent, 
split  lengthwise  into  two,  and  one  of  the  halves  goes  into  one  new 
cell  and  the  other  into  the  other;  and  they  absorb  nourishment 
and  grow  with  the  cell.  This  process  is  then  repeated  with  every 
subsequent  division,  so  that  finally  every 
cell  of  the  adult  plant  contains  chro- 
mosome material  derived  from  each  one 
of  the  parents  of  that  plant.  This  fact 
helps  to  explain  how  it  is  that  a  plant  or 
an  animal  can  resemble  either  one  of  its 
parents  in  any  detail  of  its  structure. 

At  this  point  I  will  pause  for  a  moment 
to  consider  two  matters  of  minor  impor- 
tance which  may  have  occurred  to  the 
reader.  If  the  fertilized  egg-cell  contains 
the  sum  of  the  chromosomes  of  the  two 
uniting  nuclei,  why  is  not  this  number 
doubled  again  in  the  next  generation, 
and  that  again  doubled  in  the  next,  and 
so  on  to  their  enormous  multiplication? 
The  explanation  is  simple;  at  some  period 
in  the  development  of  the  new  sexual 

Cells,    by    a   method    Which  We   need    not    FIG.  99.-A  generalized  drawing 

here    trace    in   detail,    the   number   of     of  a  !imfle  ?va|7  and  °Yule 

'  seen  in  longitudinal  section, 

Chromosomes  IS  reduced  tO  One-half.      In        showing  the  parts  concerned 
.       .  . ,  ,  ill  fertilization. 

the  second  place,  if  every  cell  contains 

within  itself  chromosome  matter  derived  from  both  parents,  and 
therefore  has  the  possibility  of  resembling  either  one  of  its  parents 
in  any  detail  of  its  structure,  what  is  it  that  determines  which  one 
it  shall  resemble?  This  we  do  not  yet  know,  but  the  probability 
would  seem  to  be  that  in  each  case  the  stronger  of  the  two  elements 
overpowers  the  other  and  reproduces  its  like. 

The  equal  contribution  of  chromosome  matter  by  male  and 
female  nuclei,  together  with  the  subsequent  regular  splitting  of 


286  The  Living  Plant 

each  chromosome  in  cell  division,  carries  an  implication  of  great 
importance  to  an  understanding  of  the  nature  of  sexual  reproduc- 
tion, namely,  it  implies  that  both  parents  contribute  exactly  alike 
to  the  characteristics  of  the  offspring,  the  selection  between  the 
double  set  of  paternal  and  maternal  characters  being  made  in  the 
course  of  development  of  the  offspring  itself.  This  view  is  dia- 


FIG.  100. — A  diagrammatic  representation  of  fertilization,  showing  the  passage  of  the  male 
nucleus  from  the  pollen  tube  into  the  egg-cell,  and  its  fusion  with  the  nucleus  thereof. 
The  black  rods  in  the  nuclei  are  chromosomes,  described  on  page  284. 

metrically  opposed  to  the  older  idea,  once  advocated  by  some 
biologists,  that  each  parent  contributes  something  the  other  does 
not;  and  it  is  obviously  quite  different  from  the  various  popular 
notions,  which,  naturally,  are  largely  erroneous. 

But  now  there  arises  this  fundamental  question.  If  the  two 
sexes  contribute  substantially  alike  to  the  offspring,  why  are  they 
not  substantially  alike  in  structure?  What  is  the  meaning  of  the 
differences  between  the  sexes?  Or,  to  go  a  stage  deeper,  why  does 
sex  exist  at  all?  Happily  these  questions  can  be  answered  with 
reasonable  certainty  through  evidence  supplied  by  a  study  of 
existing  transitions  from  the  simplest  plants,  where  sex  has  not 
yet  developed,  to  the  highest  plants  and  animals,  where  it  is  fully 
differentiated.  Thus,  there  exist  some  simply-organized  seaweeds 
which  throw  out  into  the  water  a  great  many  reproductive  cells, 
all  exactly  alike  and  provided  with  suitable  structures  for  swim- 
ming (figure  102).  These  move  towards  one  another  and  come 
together  in  couples,  which  then  fuse  completely,  uniting  their 
nuclei;  and  thus  is  formed  a  " fertilized"  cell  which  gives  origin 
to  a  new  plant  precisely  as  does  a  fertilized  egg-cell.  Obviously 
fertilization  in  this  case  occurs  between  sexual  cells  precisely  alike; 


How  Plants  Perpetuate  Their  Kinds 


287 


or,  if  one  pleases,  it  is  fertilization  without  sex.  In  the  next  place 
there  are  other  and  more  highly  organized  seaweeds  in  which  the 
reproductive  cells  given  off  into  the  water  are  of  two  different 
kinds,  although  produced  by  the  same  parent  plant.  One  kind  is 
very  much  larger,  round,  and  without  arrangements  for  locomo- 


j 


FIG.  101. — The  appearance  and  behavior  of  the  chromosomes  during  the  division  of  a 
typical  plant  cell,  as  seen,  somewhat  generalized,  in  a  series  of  optical  sections  highly 
magnified.  A  fuller  description  of  the  division  of  the  chromosomes  is  given  on  page 
284  of  this  book.  (Copied  from  Strasburger's  Textbook.) 

tion,  while  the  other  is  much  smaller,  of  elongated  shape  and  pro- 
vided with  good  swimming  organs  (figure  103).  All  of  the  move- 
ment necessary  to  bring  the  two  cells  together  in  fertilization  is 
made  by  the  active  smaller  cell,  which,  guided  no  doubt  by  some 
chemical  secretion,  swims  to  the  passive  larger  one  and  fuses 
therewith;  the  two  nuclei  then  unite  and  from  this  fertilized  cell 


288  The  Living  Plant 

a  new  plant  is  developed.    Now  the  difference  between  the  two 
cells  is  known  to  consist  in  this,  that  the  larger  possesses  a  store  of 
food  substance,  which  is  used  in  giving  a  start  to  the  new  individ- 
ual, —  the  presence  of  this  food  substance  in  the  cell  being  re- 
sponsible both  for  its  larger  size  and  its  loss 
of  locomotive  power.     The  smaller  cell,  on 
the  other  hand,  contributes  no  food  for  start- 
ing the  offspring,  but  elaborates  the  features 
concerned  in  locomotion,  thus  ensuring  that 
the  two  cells  shall  be  brought  together.    This 
difference  does  not  imply  in  the  least  that 
the  two   cells   contribute  differently  to   the 
the  left  is  a  single  one  offspring,  for  the  food  substance  supplied  by 

and  at  the  right  a  pair  ,  M.I 

in  process  of  fusion,  the  larger  cell  has  no  more  to  do  with  de- 
Chi"  termining  the  essential  characters  of  the  new 


individual  than  has  the  food  we  eat  in 
determining  our  essential  characters;  the  characters  are  de- 
termined by  the  chromatin  in  the  nuclei,  which  the  two  cells 
contribute  equally.  But  in  this  comparatively  minor  feature  of 
division  of  labor  between  the  two  kinds  of  cells  we  have  the  origin 
of  sex,  for  the  larger  cell  we  recognize  as  female  and  call  it  the  egg- 
cell,  and  the  smaller  we  recognize  as  male  and  call  it  the  sper- 
matozoid  (or  antherozoid).  This  difference  between  a  large 
immobile  egg-cell  and  a  tiny  active  male  cell,  once  established, 
persists  and  prevails  in  principle,  though  with  numerous  varia- 
tions of  detail,  throughout  all  of  the  more  highly  organized  plants, 
and  throughout  all  of  the  higher  animals,  inclusive  of  man;  and  it 
is  the  foundation  of  all  the  phenomena  of  sex. 

The  essential  characters  of  the  sexual  cells  being  thus  estab- 
lished in  these  comparatively  low  plants  in  a  degree  of  develop- 
ment as  high  as  they  ever  attain,  the  sexual  developments  in  the 
higher  plants  are  concerned  not  with  the  sexual  cells,  but  with  the 
various  accessory  structures  contributing  to  ensure  fertilization 
under  the  conditions  to  which  those  plants  are  exposed.  A  first 


How  Plants  Perpetuate  Their  Kinds 


289 


step  in  the  development  of  such  secondary  sexual  structures  is 
found  even  in  the  higher  Seaweeds  (the  Red  Algae),  in  which  the 
egg-cell  is  not  cast  loose,  as  in  the  lower  forms,  but  remains  at- 
tached to  the  parent  plant  that  forms  it  and  supplies  its  store  of 
nourishment;  while  simple  arrangements  exist  to  facilitate  the 
access  of  the  male  cell. '  But  far  more  important  is  the  step  taken 


-'• 


FIG.  103. — A  series  of  figures  illustrating  the  reproduction  of  the  common  Rockweed.  In 
the  middle  lower  part  is  the  spherical  female  (egg)  cell,  highly  magnified,  surrounded 
by  a  number  of  the  very  much  smaller  free-swimming  male  (sperm)  cells. 

in  the  simplest  land  plants,  like  the  lowly  Liverworts  and  Mosses, 
and  the  fertilization  stage  of  the  Ferns,  i.  e.,  a  stage  in  which  the 
plant  is  a  tiny  thin  leaf  pressed  close  to  the  ground.  In  all  of  these 
plants  the  very  delicate  egg-cell  and  the  subsequent  embryo  need 
protection  from  the  dryness  to  which  they  must  occasionally  be 
exposed, — and  a  protection,  of  course,  which  does  not  interfere 


290 


The  Living  Plant 


with  a  ready  fertilization.  The  structure  which  has  been  devel- 
oped in  adaptation  to  these  conditions  is  in  form  of  a  flask-shaped 
covering  to  the  buried  egg-cell  (figure  104) ;  the  end  of  the  tube 
opens  when  the  egg-cell  is  ripe  and  water  is  present,  and  exudes 
a  special  liquid  chemotropically  attractive  to  the  spermatozoids, 


FIG.  104. — A  series  of  figures  illustrating  the  reproduction  of  a  common  Fern.  The  sexual 
cells  are  borne  on  the  under  side  of  a  small  thin  leaf-like  part  close  to  the  ground. 
In  the  lower  middle  part  of  the  picture  is  a  squarish  egg-cell  with  prominent  nucleus, 
buried  in  chlorophyllous  tissue,  and  covered  with  an  elongated-tubular  structure, 
down  the  cavity  of  which  a  spiral-shaped  male  cell  is  proceeding  to  unite  with  the 
egg-cell. 

which  then  swim  towards  and  down  the  neck  to  the  egg-cell.  But 
such  plants  are  as  obviously  dependent  upon  water  for  fertiliza- 
tion as  are  the  Seaweeds;  and  hence  they  are  confined  to  places 
habitually  wet,  or  must  grow  so  close  to  the  ground  that  fertiliza- 
tion can  be  effected  during  flooding  by  rains.  Still  another  step, 
but  this  time  the  final  one  for  plants,  in  the  evolution  of  secondary 


How  Plants  Perpetuate  Their  Kinds  291 

sexual  structures,  is  taken  in  the  flowering  plants,  which  carry 
their  sexual  parts  high  up  in  the  air.  In  consequence  of  the  greater 
dryness  of  that  situation,  they  have  had  to  bury  their  sexual  parts 
far  more  deeply  (viz.,  deep  inside  the  ovules  and  anthers),  and 
have  had  to  abandon  the  free-swimming  sperm  cell  of  all  the 
lower  kinds  and  replace  it  by  the  growing  pollen-tube,  which 
carries  the  male  cell  to  the  female  cell  in  the  way  we  have  already 
described.  In  a  word,  the  structures  developed  adaptively  in  re- 
sponse to  the  conditions  of  protection  and  fertilization  in  these 
highest  plants  are  the  stamens  and  pistils,  the  essential  parts  of  the 
flower  which  we  have  already  described.  But  all  such  structures, 
like  all  of  the  sexual  parts  and  adaptations  developed  by  animals, 
are  in  reality  secondary,  being  merely  arrangements  to  enable  the 
male  cell  to  effect  fertilization  and  the  female  cell  to  receive  it. 
The  central  and  essential  feature  of  fertilization  and  sexual  union, 
viz.,  the  union  of  nuclei  carrying  the  hereditary  qualities  of  two 
parents ;  and  the  central  and  essential  feature  of  difference  between 
the  sexes,  viz.,  a  division  of  labor  between  the  two  parents; — 
these  remain  the  same  throughout  the  plant  and  animal  kingdoms 
from  the  lowliest  of  the  seaweeds  up  to  man  himself. 

Sex,  therefore,  does  not  arise  in  any  essential  difference  of  rela- 
tion of  the  two  parents  to  offspring,  but  in  a  minor  and  mechanical 
matter  of  division  of  labor  between  the  sexual  cells,  involving 
secondary  differences  in  various  accessory  structures.  Sex,  so  to 
speak,  is  not  a  matter  of  method  but  of  mechanism,  and  exists  not 
for  the  sake  of  the  formation  of  offspring  but  for  giving  it  a  more 
certain  and  better  start  in  its  life.  That  cell,  structure,  or  in- 
dividual, which  is  devoted  to  nourishing  and  protecting  the  young 
individual  formed  as  a  result  of  fertilization  we  call  female:  that 
cell,  structure,  or  individual,  which  is  devoted  to  bringing  the  two 
cells  together  for  fertilization  we  call  male.  This  difference  is  the 
central  feature  of  all  the  phenomena  of  sex,  although  worked  out 
with  infinite  variety  of  detail  and  more  or  less  interlocked  with 
other  considerations;  and  it  explains  not  only  sexual  structures  in 


292  The  Living  Plant 

plants,  but  in  animals  also,  including  man.  With  man,  indeed, 
the  principle  that  the  female  is  the  receptive  and  protective 
element,  and  the  male  the  aggressive  element,  is  not  limited  to 
physical  structure  alone,  but  shows  its  influence  in  some  of  the 
profoundest  facts  of  his  actions,  thoughts,  laws,  and  social  cus- 
toms. 

At  this  point  the  reader  may  demur  to  this  explanation  of  sex 
on  the  ground  that  it  seems  too  superficial  for  the  profundity  of 
the  phenomena.  But  one  should  take  care  not  to  extend  to  all 
Nature  conceptions  derived  alone  from  mankind,  where  all  sexual 
matters  are  vastly  exaggerated  in  apparent  importance  by  socio- 
logical considerations.  In  Nature  at  large  sexual  differences  are 
prominent  rather  than  profound.  Even  in  plants  that  are  highest 
in  the  evolutionary  scale,  sexual  differences  never  affect  the  plant- 
structure  very  far  away  from  the  pistils  and  stamens;  and  all  of 
the  remainder  of  the  bulk  of  the  plant  has  nothing  whatever  to  do 
with  sex,  but  is  strictly  nonsexual  or  asexual.  Even  the  occasional 
cases,  described  by  the  term  dioecious,  where  one  plant  bears  only 
pistils  and  another  only  stamens,  is  no  real  exception,  though  we 
often  describe  these  plants,  very  naturally,  as  male  and  female 
respectively.  The  individual,  therefore,  in  plants  is  sexual  only 
in  limited  spots,  it  is  never  sexual  as  a  whole.  The  same  is  true 
of  the  simpler  animals,  but  in  the  more  highly  organized  species 
it  is  somewhat  different,  for  in  them  each  individual  bears  only 
one  kind  of  sex  cells,  and  has  only  one  kind  of  sex  organs;  while 
the  high  specialization  of  these  parts  affects  somewhat  the  whole 
individual  so  that  we  distinguish  male  and  female  individuals. 
But  even  in  mankind  the  structural  differences  between  the  sexes 
are  insignificant  as  compared  with  the  structural  resemblances 
between  them. 

Whatever  else  my  discussion  of  sexual  reproduction  may  have 
meant  to  the  reader,  it  will  at  least  have  demonstrated  this,  that 
sexual  reproduction  is  a  far  more  complicated  process  than 
asexual,  involving  the  construction  and  manipulation  of  adapta- 


How  Plants  Perpetuate  Their  Kinds  293 

tions  wholly  needless  in  the  asexual  methods.  Yet  plants  can 
reproduce  perfectly  well  by  the  direct  and  simple  asexual  methods. 
In  what,  then,  consists  the  superiority  of  sexual  reproduction  that 
plants  and  animals  should  not  only  take  so  great  trouble  to  secure 
it,  but  should  even  abandon  in  the  higher  forms  the  asexual 
method  entirely?  Many  answers  have  been  proposed  for  this 
question,  and  we  do  not  yet  know  the  truth  with  certainty;  but 
the  most  probable  explanation  is  derived  from  the  fact  that  sex- 
ually produced  individuals  are  usually  more  variable,  adaptable, 
and  vigorous  than  those  asexually  produced,  and  hence  in  the 
long  run  overcome  the  latter  in  the  struggle  for  existence,  and  sur- 
vive while  the  others  die  out.  An  asexually  generated  individual 
is  naturally  no  more  than  a  chip  of  the  old  block,  and  can  differ 
but  little  therefrom,  while  one  sexually  produced  has  the  possi- 
bility of  combining  the  good  qualities  from  two  parents.  Now  if 
conditions  surrounding  plants  were  unchanging,  and  all  plants 
were  adapted  the  best  possible  thereto,  then  the  asexual  method 
might  be  really  the  better;  but  the  conditions  of  the  world  are 
continually  changing,  and  therefore  those  animals  and  plants 
which  possess  the  most  variability  and  adaptability  have  the 
best  chance  of  maintaining  themselves  therein.  This  is  the  reason, 
I  think,  why  sexual  reproduction,  despite  its  complications,  has 
displaced  the  far  simpler  asexual  kind. 

The  greater  constancy  of  characteristics  usually  distinctive  of 
asexual  reproduction,  in  comparison  with  the  greater  variability 
associated  with  the  sexual  type,  has  some  very  practical  conse- 
quences. Thus,  as  everybody  knows,  we  can  reproduce  Bartlett 
pear  trees  by  the  asexual  method  of  grafting,  and  keep  the  fine 
qualities  of  the  fruit;  but  if  we  propagate  them  by  seeds,  which 
of  course  are  formed  only  as  a  result  of  fertilization,  we  do  not  get 
Bartlett  pears  at  all,  but  just  a  plain  mongrel  variety;  and  the 
same  thing  is  true  of  many  other  kinds  of  highly  perfected  garden 
plants.  This  principle  is  so  well  understood  by  gardeners  that 
whenever  they  seek  to  secure  new  forms  of  plants,  they  make  use 


294  The  Living  Plant 

of  seed  propagation;  and  whenever  they  have  obtained  a  specially 
good  kind,  they  try  to  preserve  it  by  propagating  it  asexually. 
But  we  are  verging  over  to  the  subject  of  plant  breeding,  which 
is  a  matter  so  important  that  it  must  later  receive  a  chapter  to 
itself. 

We  must  here  turn  back  to  fertilization  in  order  to  consider 
another  important  phenomenon  in  connection  therewith.  Al- 
though in  the  higher  plants  both  pistils  and  stamens  are  usually 
associated  closely  together  within  one  flower,  it  is  only  excep- 
tionally the  case  that  egg-cells  are  fertilized  by  pollen  from  that 
same  flower.  On  the  contrary  there  exist  the  most  elaborate  ar- 
rangements adapted  to  prevent  such  a  close  fertilization,  and 
ensure  that  the  sex  cells  which  unite  shall  come  from  different 
flowers,  and  usually  indeed  from  different  plants.  It  is  in  adapta- 
tion to  such  cross  pollination  that  plants  have  developed  the  more 
conspicuous  features  of  the  flower,  the  nectar,  odor  and  showy 
corolla  in  particular,  as  will  appear  in  the  following  chapter  which 
is  wholly  devoted  to  this  interesting  subject.  Indeed,  in  some 
plants  the  arrangement  is  such,  notably  where  the  stamens  and 
pistils  are  borne  on  quite  different  plants,  that  close  fertilization 
is  not  even  possible;  and  this  arrangement  is  universal  among  the 
higher  animals.  Now  it  is  quite  plain  that  the  fertilization  of  an 
ovule  by  pollen  from  the  very  same  flower  would  be  vastly  easier 
of  accomplishment  than  is  the  elaborate  cross  fertilization, — re- 
quiring no  more,  indeed,  that  a  simple  turning  of  the  stamen  over 
against  the  stigma,  when  the  growth  of  the  pollen-tube  would 
accomplish  the  rest ;  and  the  fact  that  plants  not  only  choose  the 
most  difficult  method,  but  also  even  abandon  in  the  higher  forms 
the  very  possibility  of  the  simpler,  shows  quite  conclusively  that 
cross  fertilization  has  some  great  merit  above  close  fertilization. 
And  the  reason  for  the  superiority  is  not  difficult  to  find.  It  was 
first  indicated  by  the  experiments  of  Darwin,  who  showed  that 
the  progeny  resulting  from  cross  fertilization  can  be  more  vigorous 
and  numerous  than  those  from  close  fertilization;  and  the  same 


How  Plants  Perpetuate  Their  Kinds  295 

thing  is  confirmed  in  general  by  the  experience  of  animal  breeders 
who  know  that  the  parents  must  be  not  too  nearly  related  if  the 
offspring  are  to  be  of  the  best.  Indeed,  it  is  evident  that  if  pollen 
and  ovule  belong  to  the  same  flower  or  even  the  same  plant,  we 
have  a  near  approach  to  vegetative  reproduction;  while  the  full 
value  of  fertilization  can  be  realized  only  when  the  uniting  sex 
cells  come  from  different  individuals.  In  a  word,  cross  fertiliza- 
tion has  much  the  same  advantage  over  close  fertilization  that 
fertilization  has  over  asexual  reproduction;  and  this  advantage 
has  sufficed  to  enable  the  kinds  which  have  developed  it  to 
triumph  over  those  which  have  not.  If  it  seems  to  the  reader  that 
in  cases  like  these  Nature  goes  to  a  trouble  out  of  all  proportion  to 
the  advantage  attained,  I  would  remind  him  that  life  is  a  kind  of 
race  in  which  only  a  few  can  be  winners;  and  that  no  effort  can 
be  too  great  to  put  forth  when  to  live  is  the  prize  and  to  lose  is 
death. 

There  is,  furthermore,  still  another  matter  of  the  highest  impor- 
tance which  must  receive  our  attention  in  connection  with  ferti- 
lization. Everybody  has  heard  something  about  Mendel's  Law, 
though  it  is  not,  as  yet,  widely  understood.  Mendel  was  an  Aus- 
trian monk,  who,  in  his  monastery  garden,  a  half  century  ago, 
began  experimenting  systematically  upon  heredity,  and  thereby 
discovered  the  most  important  facts  we  yet  know  about  that  fun- 
damental subject.  In  order  that  the  characters  transmitted  by 
each  parent  might  be  distinguishable  in  their  offspring,  he  selected 
as  parents  not  plants  of  the  same  variety,  but  of  distinct  varieties 
differing  markedly  in  some  given  features;  and  furthermore,  in 
order  to  avoid  the  complications  caused  by  cross  fertilization,  he 
chose  kinds  which  fertilize  themselves,  as  do  a  number  of  culti- 
vated species.  Accordingly,  taking  Peas,  which  fulfil  these  con- 
ditions, he  bred  together  a  kind  with  green  cotyledons  and  an- 
other with  yellow  cotyledons.  The  resulting  offspring  were  of 
course  hybrids,  but  their  cotyledons  were  not,  as  one  would 
expect,  greenish-yellow,  or  yellowish-green,  but  were  all  yellow 


296  The  Living  Plant 

like  those  of  one  of  the  parents  and  quite  free  from  the  green  of 
the  other.  These  hybrids,  when  grown,  were  also  self-fertilized, 
and  produced  a  large  number  of  offspring;  and  as  a  result  a  re- 
markable fact  came  to  light, — namely,  that  although  approxi- 
mately three-fourths  (75%)  of  these  plants  possessed  yellow 
cotyledons,  one-fourth  (25%)  had  green  cotyledons  just  like  those 
of  one  of  the  grandparents.  Furthermore,  when  these  green- 
cotyledoned  forms  were  propagated  by  self-fertilization,  all  of 
their  numerous  offspring  had  green  cotyledons,  and  never  yel- 
low, which  color  was  thus  permanently  bred  out  of  these  plants 
and  their  descendants.  But  when  the  75%  of  yellow-cotyledoned 
forms  were  self-fertilized,  their  offspring  gave  this  striking  result, 
that  one-third  of  them  (and  therefore  one-fourth,  or  25%,  of  the 
entire  original  number  in  this  generation)  produced  only  yellow- 
cotyledoned  kinds,  as  did  their  offspring,  and  theirs  again  indefi- 
nitely, the  green  being  thus  bred  permanently  out  of  these  forms 
and  their  descendants.  But  the  remaining  two-thirds  of  the  yellow 
kind  (forming  half,  or  50%,  of  the  total  number),  acted,  when 
self-fertilized,  precisely  as  their  parents  had  done,  producing  25% 
green,  and  75%  yellow  kinds  of  which  latter  25%  bred  perma- 
nently yellow;  and  the  same  thing  was  repeated  in  the  next  genera- 
tion, and  so  on  without  limit.  The  method  of  this  distribution  of 
characters  in  the  offspring  is  shown  graphically  in  the  accom- 
panying diagram  (figure  105),  in  which,  however,  the  exigencies 
of  printing  forbid  the  use  of  color,  for  which  reason  the  yellow 
cotyledons  are  represented  by  the  solid  black  circles,  and  green 
by  the  white  ones.  Moreover,  the  first  generation,  (and  half 
of  the  later  individuals)  though  themselves  possessing  yellow 
cotyledons  as  a  visible  or  so-called  dominant  character,  have 
obviously  the  power  of  transmitting  the  green  as  an  invisible  or 
recessive  character;  and  this  fact  is  represented  in  the  diagram  by 
giving  to  those  individuals  a  white  center.  It  is  not  yet  wholly 
clear,  by  the  way,  why  Peas  which  have  the  power  of  transmitting 
both  yellow  and  green  cotyledons  should  always  have  yellow  ones; 


How  Plants  Perpetuate  Their  Kinds  297 

but  such  is  the  fact,  and  such  dominance  of  one  character  over 
another  is  always  a  feature  of  Mendelian  inheritance. 

Now  this  remarkable  distribution  of  contrasting  characters  is 
true  not  of  the  cotyledon  color  of  Peas  alone,  but  of  their  flower 
colors,  their  height,  and  other  characteristics;  and  not  of  Peas 
alone  but  of  innumerable  other  plants,  and  likewise  of  the  most 
diverse  animals,  in  the  most  diverse  characters.  Moreover,  while 
discovered  and  most  conspicuous  in  hybrids,  it  is  also  true  in  prin- 
ciple of  all  kinds  which  breed  together;  and  while  its  mathe- 
matical basis  can  be  traced  clearly  only  in  self-fertilizing  forms, 
it  holds  true,  though  of  course  with  proportional  complications,  in 


°L 


I 

000 


ri  i  i      i      i  i      i  n  i      i  PI  i 
oooo    ooo*    ooo«    •••• 

FIG.  105. — A  diagram  illustrating  Mendelian  inheritance.    It  is  fully  explained  in  the  text. 

cross-fertilized  forms.  There  is,  indeed,  no  longer  any  doubt  that 
it  represents  a  very  wide  spread  principle  of  heredity.  Indeed, 
were  it  not  for  the  numerous  complications  introduced  by  the 
complexity  of  life-phenomena,  it  would  probably  be  found  to  hold 
true  universally. 

Mendel's  discoveries  have  thus  shown  not  only  that  heredity  is 
correlated  with  a  certain  mathematical  principle,  but  also  that 
any  undesirable  feature  can  be  rapidly  and  surely  bred  out  of  a 
race,  and  need  not  require  the  slow  process  of  dilution  out,  so  to 
speak,  as  was  formerly  supposed. 

There  is  one  fact  about  this  Mendelian  distribution  of  characters 
which  will  illuminate  the  whole  subject  greatly  if  the  reader  will 


298  The  Living  Plant 

but  grasp  it  clearly  at  the  start, — namely,  that  it  applies  only  to 
single  individual  characters,  never  to  large  collections  of  them, 
and  much  less  to  the  whole  aggregate  of  characters  displayed  by 
each  parent.  Each  of  the  many  characters  transmitted  by  parents 
to  offspring  conforms  to  this  general  principle,  but  they  are 
transmitted  to  no  two  of  the  offspring  in  the  same  combinations. 
The  offspring  are  thus  like  the  different  patterns  displayed  by  the 
same  pieces  of  colored  glass  in  the  turning  of  a  kaleidoscope;  and 
this  is  very  well  exemplified  in  the  familiar  cases  among  mankind, 
where  the  characteristics  of  two  parents  reappear  in  the  most 
different  combinations  in  their  children.  Nevertheless,  in  self- 
fertilized  races  of  plants  it  is  possible  to  fix  permanently  certain 
combinations  of  characters  by  breeding  out  their  opposites,  and 
then  these  combinations  repeat  themselves  with  the  greatest 
fidelity.  Such  combinations  we  shall  meet  again  in  our  chapter  on 
evolution  under  the  name  of  genotypes. 

Finally,  we  consider  for  a  moment  the  explanation  of  the  re- 
markable mathematical  arrangement  revealed  by  Mendel's  Law. 
In  the  first  place  it  is  plain  that  each  definite  character  of  the 
adult  individual  is  represented  by  some  kind  of  determiner  in  the 
germ  cells  (i.  e.  egg-cells  and  sperm  cells),  and  that  any  individual 
is  a  mosaic  of  characters  of  which  the  germ  cells  (or,  rather,  their 
chromosomes),  are  collections  of  the  corresponding  determiners. 
Also,  the  facts  show  that,  in  harmony  with  the  behavior  of 
the  chromosomes  in  cell-division  and  fertilization,  while  every 
body  cell  of  each  individual  contains  two  determiners  for  each 
character  (one  derived  from  each  parent),  every  germ  cell,  because 
of  the  reduction  division  earlier  mentioned  (page  285),  contains 
only  one  or  the  other  of  these  determiners  and  never  both,  a  fact 
expressed  in  the  phrase  " purity  of  the  germ  cells."  Thus  in  our 
Peas,  earlier  instanced,  each  of  the  male  cells,  and  also  each  of  the 
female  cells,  contain  a  determiner  for  either  a  yellow  or  a  green 
cotyledon,  but  never  both.  Now  if  large  numbers  of  such  male  and 
female  germ  cells  are  allowed  to  come  together  at  hap-hazard,  as  in 


How  Plants  Perpetuate  Their  Kinds  299 

fact  they  do  in  fertilization,  then  a  certain  proportion  of  those  con- 
taining yellow  determiners  will  unite  with  others  containing  yellows, 
and  green  will  be  excluded  from  the  resulting  individuals;  a  certain 
proportion  of  greens  will  unite  with  greens,  thus  excluding  yellow; 
a  certain  proportion  of  greens  will  unite  with  yellows,  and  a  cer- 
tain proportion  of  yellows  will  unite  with  greens.  These  propor- 
tions (allowing  for  the  fact  that  the  yellow-green  and  the  green- 
yellow  are  indistinguishable)  will  be  precisely  those  actually 
found  by  the  law  to  exist,  as  above  described,  and  precisely  those 
shown  in  our  diagram. 

There  remain  a  few  miscellaneous  matters,  connected  with 
reproduction,  which  must  be  considered  before  this  chapter  can 
be  brought  to  a  close. 

It  is  almost  invariably  the  case  that  an  egg-cell  must  be  fer- 
tilized by  a  male  cell  before  it  will  grow  to  a  new  plant,  but  a  few 
exceptions  are  known.  In  some  few  plants  of  the  Composite 
family,  and  in  the  Plant  Lice  among  Insects,  the  egg-cells  grow 
directly  into  new  individuals  without  any  fertilization  or  other 
connection  with  the  males, — a  phenomenon  appropriately  called 
parthenogenesis.  It  is  a  kind  of  asexual  growth  of  the  egg-cell, 
comparable  with  the  growth  of  a  tiny  bud;  and  possibly  the  es- 
sential meaning  of  the  process  lies  in  its  asexual  character.  It  is 
conceivable  that  these  particular  kinds  of  plants  and  animals  have 
reached  the  highest  practicable  stage  of  adaptation  to  the  con- 
ditions around  them,  in  which  case  it  would  be  natural  for  them 
to  preserve  their  characteristics  unchanged  by  resorting  to  asexual 
propagation,  using  the  method  which  entails  least  disturbance  to 
existent  structures  and  habits. 

In  my  account  of  fertilization  I  showed  that  the  pollen  grain 
when  it  enters  the  embryo  sac  contains  two  separate  nuclei,  only 
one  of  which  is  needed  to  fertilize  the  egg-cell.  The  fate  of  the 
other  is  most  peculiar,  for  in  some  plants  at  least,  and  probably 
as  a  rule,  it  fuses  with  a  nucleus  belonging  to  the  embryo  sac 
itself.  The  resultant  cell  grows  into  the  mass  of  food  substance 


300  The  Living  Plant 

(the  endosperm),  devoted  to  nourishing  the  embryo  in  its  growth. 
At  first  sight  it  would  appear  as  if  the  embryo  and  the  endosperm 
were  brothers,  so  to  speak,  one  of  which  later  feeds  cannibalis- 
tically  upon  the  other;  but  probably  this  is  not  its  meaning.  It  is 
more  likely  that  the  fusion  is  simply  a  method  of  providing  a 
stimulus  for  the"  formation  of  the  endosperm; — a  signal,  so  to 
speak,  to  the  waiting  embryo  sac  nucleus  that  fertilization  has 
really  been  accomplished  and  therefore  the  endosperm  will  be 
needed, — for  the  endosperm  does  not  form  unless  fertilization  is 
accomplished.  The  matter,  however,  would  have  a  purely 
scientific  interest  were  it  not  for  a  rather  well-known  phenomenon 
it  explains.  Most  people  are  aware  that  some  varieties  of  corn 
produce  red  ears  while  others  have  white  ones,  and  that  some- 
times, where  the  two  kinds  grow  together,  red  grains  appear  on 
the  white  ears.  This  has  long  been  known  to  be  due  in  some  way 
to  the  influence  of  the  pollen  of  the  red  kind  upon  the  white  ears, 
but  the  remarkable  matter  about  it  was  this,  that  the  color  was  not 
in  the  embryo,  where  its  presence  would  be  natural,  but  in  a  part 
of  the  grain  which  was  apparently  made  by  the  white  parent. 
Here  was  a  case  in  which  the  male  parent  not  only  fertilized  the 
egg-cell,  but  even  seemed  to  affect  the  structure  of  the  female 
parent,  a  phenomenon  called  xenia  in  plants,  and  often  reported, 
though  never  confirmed,  among  animals.  But  the  discovery  of 
this  double  fertilization  removed  all  mystery  from  the  matter,  for 
the  color  in  the  red  grains  resides  wholly  in  the  endosperm,  which 
is  a  kind  of  a  hybrid  between  the  male  and  female  parents,  sharing 
in  the  characters  of  both. 

As  our  chapter  on  Protoplasm  showed,  individuals  tend  to  wear 
out  and  die  when  their  protoplasm  repeats  longtime  the  same 
function;  but  they  can  live  potentially  forever  if  the  protoplasm 
can  change  periodically  its  internal  arrangement, — can  go,  so  to 
speak,  into  the  melting  pot,  and  be  cast  anew.  Now  there  is  no 
more  effective  remelting  than  that  accompanying  sexual  reproduc- 
tion, for  a  greater  change  in  the  constitution  of  the  protoplasm 


How  Plants  Perpetuate  Their  Kinds  301 

could  hardly  be  imagined  than  that  which  occurs  through  the 
commingling  of  two  different  cells.  At  all  events  fertilization  is 
always  followed,  especially  in  animals,  by  that  display  of  vigor 
and  activity  which  we  call  youth  or  juvenescence,  whereby  the 
racial  vigor  is  periodically  renewed  in  each  generation.  Indeed, 
so  prominent  and  advantageous  is  this  rejuvenescence  that  some 
biologists  have  thought  to  find  therein  the  chief  utility  of  sexual 
reproduction.  Perhaps  it  does  indeed  play  some  part,  for  sexual 
reproduction,  like  many  other  physiological  processes,  is  probably 
not  the  expression  of  a  single  factor,  but  the  resultant  of  the  co- 
operation of  several. 

Replacement  of  the  individuals  which  must  die  is  no  doubt  the 
first  meaning  of  reproduction,  but  therewith  is  often  associated 
the  idea  of  multiplication  in  number.  Multiplication,  however, 
is  more  seeming  than  real,  as  shown  by  this  fact,  that  in  general 
any  kind  of  animal  or  plant,  no  matter  how  numerous  its  off- 
spring, does  not  alter  its  numbers  appreciably  from  one  year  to 
another.  Thus,  in  general,  there  are  no  more  Mushrooms,  Dan- 
delions, or  Robins  in  a  given  county  this  year  than  last,  and  the 
numbers  of  each  kind  remain  for  decades  substantially  stationary. 
Even  the  occasional  exceptions  caused  by  the  introduction  of  new 
weeds  or  animal  pests,  or  by  the  expansion  of  man  himself,  is  no 
real  exception,  for  after  a  time  these  also  attain  a  condition  of 
numerical  stability.  Hence,  the  offspring  formed  by  animals  and 
plants  do  not  in  general  increase  their  numbers,  but  simply  make 
up  for  losses.  In  reproduction,  therefore,  multiplication  is  sub- 
ordinate to  continuance  of  the  kind. 

Reproduction,  as  we  have  seen,  is  essentially  Nature's  method 
of  continuing  the  kinds  of  plants  or  of  animals  as  the  individuals 
perish.  This  being  true  it  follows  that  if  the  individuals  were 
immortal,  there  would  be  no  need  for  reproduction,  after  once 
the  world  was  fully  populated.  This  view  receives  confirmation 
from  the  balance  which  exists  between  the  vegetative  prosperity 
of  the  individual  and  its  reproduction, — anything  favoring  the  one 


302  The  Living  Plant 

tending  to  check  the  other.  Thus,  many  simple  forms  will  not 
form  reproductive  parts  so  long  as  the  solutions  in  which  they  live 
contain  plenty  of  food,  and  the  other  conditions  are  favorable; 
and  it  is  only  when  they  begin  to  feel  the  effects  of  insufficient 
food  or  temperature  that  they  will  begin  to  form  reproductive 
bodies  at  all.  Even  in  the  higher  plants  the  same  principle  holds, 
and  all  farmers  know  that  when  soils  are  too  heavily  fertilized 
many  plants  tend  to  "run  to  leaf,"  and  flower  very  badly,  while 
there  are  plants  of  our  greenhouses  (e.  g.  Bougainvillaea)  which 
must  actually  be  partially  starved  before  they  will  form  any 
flowers.  The  same  principle  holds  good  with  animals ;  they  must 
not  be  too  highly  pampered  and  fed,  else  their  reproductive  powers 
suffer.  I  believe  that  we  have  the  operation  of  the  same  principle 
upon  a  very  large  scale  among  mankind  in  the  fall  of  the  birth-rate 
amongst  the  most  highly  civilized  races,  and  the  highest  classes 
of  each  race.  In  general  the  birth-rate  is  lowest  where  the  hygienic 
and  other  conditions  are  most  favorable  for  the  preservation  and 
comfort  of  the  individual,  and  the  birth-rate  grows  higher  among 
peoples  and  classes  in  which  the  conditions  of  life  are  markedly 
harder.  Harder  conditions  of  life  presage  an  earlier  end  to  the 
life  of  the  individual,  and  Nature  seems  to  have  adopted  their 
presence  as  the  stimulus  or  signal  for  setting  the  reproductive 
apparatus  more  actively  at  work. 


CHAPTER  XII 

THE   MANY   REMARKABLE   ARRANGEMENTS   BY 
WHICH  PLANTS  SECURE  UNION  OF  THE  SEXES 

Cross  pollination;  Flowers 

]HE  preceding  chapter  should  have  made  it  quite  clear 
that  plants  possess  sex;  that  this  is  the  same,  both 
female  and  male,  as  it  is  among  animals;  that  a  union 
of  the  two  is  generally  needful  for  the  production  of 
offspring;  and  that  the  offspring  is  usually  better  in  quality  if  the 
uniting  sex  cells  are  derived  from  separate  parent  plants.  But 
the  union  of  sex  cells  from  separate  parents  presents  a  difficult 
problem  to  those  plants  which,  including  all  of  the  higher  and 
more  familiar  kinds,  are  sedentary,  and  therefore  unable  to  come 
together  by  their  own  powers  of  locomotion,  as  animals,  and  indeed 
some  of  the  water  plants,  so  readily  do.  Specifically,  their  prob- 
blem  is  this,  to  secure  the  transfer  of  the  small  and  light  pollen 
grains,  which  contain  the  male  cells,  from  the  anthers  of  one 
plant  across  some  space  to  the  stigmas,  which  give  access  to 
the  female  cells  contained  in  the  ovules,  of  another,  after  which, 
of  course,  fertilization  proceeds  by  the  methods  already  described 
very  fully  in  the  chapter  on  Reproduction.  The  problem  of  cross 
fertilization,  therefore,  resolves  itself  in  such  plants  into  one  of 
cross  pollination,  which  is  effected  by  methods  that  we  must  now 
consider  in  detail. 

Let  us  first  dispose  of  the  simpler  methods  displayed  by  the 
Water  plants,  which  in  some  cases  possess  an  animal-like  power 
of  independent  locomotion  by  swimming,  particularly  in  their 


3°4 


The  Living  Plant 


male  cells.    In  most  of  the  Seaweeds  (or  Algae),  of  both  salt  and 
fresh  water,  both  kinds  of  sexual  cells  are  cast  out  into  the  water, 
where   those   from    different    plants   become    completely    com- 
mingled, especially  under  action  of  currents,  waves,  and  the 
power  of  the  male   cells  to   swim  freely 
about;  and  apparently  mere  chance  under 
these   conditions   is   enough   to  ensure   a 
sufficiency  of  crossing  between  different  par- 
ents, although,  for  all  we  know,  elaborate 
physiological     arrangements,     comparable 
with  some  of  those  which  will  presently 
be  described  for  the  higher  plants,  may 
exist  to  prevent  union  of  sex  cells  pro- 
duced by  the  same  plant.    Such  arrange- 
ments, indeed,  are  known  to  occur  in  the 
higher   kinds   of   plants   fertilized   in  the 
FIG.  106.— Ceils  from  dis-  water,  notably  the  Ferns,  where  the  male 
f^lT^y^ni.  *nd  female  sex  cells  produced  on  the  same 
fied,  showing  the  forma-  plant  ripen  at  different  tunes.    Again,  in 

tion  of  a  fertilization  tube. 

(From  the  Chicago  Text-  some  other  kinds  of  low  Water  plants,  whose 
habits  are  such  that  the  many  long  threads 

of  which  their  bodies  consist  live  tangled  or  felted  together,  slender 
tubular  projections  (a  kind  of  premonition  of  the  pollen-tube), 
grow  out  and  connect  one  thread  with  another  (figure  106) ;  and 
through  the  passage  thus  formed  the  contents  of  one  cell  can 
unite  with  another  in  cross  fertilization,  though  plenty  of  cases 
are  known  in  which  the  same  method  is  used  in  the  fertilization  of 
one  cell  by  another  within  the  same  thread. 

While  the  Seaweeds,  or  Algae,  are  the  distinctive  plants  of  the 
waters,  a  good  many  kinds  of  Flowering  plants,  originally  in- 
habitants of  the  land,  have  been  forced  into  life  in  the  water, 
developing,  of  course,  appropriate  adaptations  thereto.  Of  these, 
the  conspicuous  kinds,  like  the  Water  Lilies,  secure  their  cross 
pollination  by  the  very  same  methods  as  the  showy-flowered 


Arrangements  for  Securing  Union  of  Sexes        305 


plants  of  the  land,  which  we  shall  consider  a  few  pages  later;  but 
a  great  many  others  of  simpler  sort,  including  especially  the 
lowlier  Waterweeds,  cast  their  suitably-protected  pollen  out  into 
the  water,  to  be  drifted  about  by  the  currents  until  it  reaches  the 
stigmas.  In  some  kinds,  as  in 
most  of  the  Eel-grasses,  where 
the  pollen  is  thread-like  in 
shape,  the  pollination  occurs 
under  water;  but  in  others,  for 
example  the  Freshwater  Eel- 
grass,  Vallisneria  spiralis  (figure 
107),  it  takes  place  on  the  sur- 
face, to  which  the  staminate 
flowers  rise  from  their  place  of 
formation,  and  on  which  floats 
the  ripe  ovary  with  widely- 
spread  stigmas.  Then  the 
movements  of  the  surface  cur- 
rents, with  aid  of  the  wind, 
bring  the  pollen  sooner  or  later 
to  the  stigma. 

But   far  more   striking    and 
important  are  the  adaptations 

to    CrOSS     pollination     found     in    FIG.   107.— Cross  pollination  in  the  Water- 

plants  that  live  out  on  the  land, 
including  the  kinds  with  which 
we  are  most  familiar.  These, 
having  no  power  at  all  of  loco- 
motion, have  had  to  secure  the 
transport  of  their  pollen  in  some  different  way  and  that  way  con- 
sists in  the  utilization,  by  aid  of  suitable  adaptive  mechanisms  and 
methods,  of  such  motive  agencies  as  happen  to  exist  in  the  world 
around.  Now  of  all  such  agencies,  the  most  ubiquitous  and  the 
easiest  to  utilize  is  the  wind.  Accordingly  wind  pollination  prevails 


weed,  Vallisneria  spiralis,  which  is  shown 
about  one-third  the  natural  size.  The 
staminate  flowers  may  be  seen  rising  to 
the  surface,  where  they  open  and  are 
drifted  about  until  their  stamens  come 
into  contact  with  the  long-stalked  float- 
ing pistillate  flowers.  (Copied,  somewhat 
simplified,  from  Kerner's  Pflanzenleben.) 


3o6 


The  Living  Plant 


in  a  good  many  plants,  especially  trees,  and,  in  lesser  degree, 
shrubs,  for  these  are  most  exposed  to  the  sweep  of  the  winds ;  while 
it  is  rare  in  herbs  and  confined  mostly  to  those  that  grow  in  fully 
exposed  places.  In  such  plants  the  smooth  light  pollen  grains  often 
possess  bladders,  or  wings  providing  more  surface  for  action  of  the 
wind  (figure  108),  while,  moreover  they  are  produced  in  vast 


FIG.  108. — Typical  pollen  grains,  highly  magnified.  On  the  left  next  above  the  bottom 
row,  are  three  from  the  Pine,  showing  the  attached  bladders.  The  very  rough  kinds, 
especially  those  of  the  upper  row,  are  carried  by  insects,  to  whose  hairy  bodies  they  are 
thus  adapted  to  cling.  (Reduced  from  Kerner's  Pflanzenleberi) . 

quantities  to  compensate  for  the  inevitable  waste  inseparable 
from  this  method.  For  this  reason  the  staminate  blossoms  of  such 
plants  far  outnumber  the  pistillate,  as  witnessed  by  the  fact  that 
long  hanging  staminate  catkins,  from  which  one  can  dislodge  a 
cloud  of  fine  yellow  dust  by  a  touch,  are  familiar  to  everybody  in 
Birches,  Alders,  Poplars,  Butternuts,  and  other  trees  in  the 
spring;  while  the  pistillate  blossoms,  which  commonly  occur  on 


Arrangements  for  Securing  Union  of  Sexes        307 


separate  plants,  or  at  least  in  separate 
flowers,  are  comparatively  so  incon- 
spicuous that  they  scarcely  are  known 
at  all,  and  need  a  considerable  search  to 
reveal  them.  The  relative  conspicuous- 
ness  and  abundance  of  the  two  kinds  of 
blossoms  are  typically  shown  by  the 
Hazel  (figure  109).  When  found,  how- 
ever, these  pistils  are  distinguished  by 
large,  and  usually  branched  or  hairy, 
stigmas, — an  obvious  net  spread  for  the 
stoppage  of  the  wind-drifted  pollen. 
Thus  the  "silk"  of  the  Corn,  wherein 
each  strand  is  a  style  along  which  grows 
a  pollen-tube  to  each  grain,  stands  out 
from  the  young  ears  when  their  grains 
are  ready  for  fertilization,  as  a  feathery 
cluster  of  styles  and  stigmas,  which 
catch  the  pollen  carried  by  wind  from 
the  staminate  tassels,  though  later  when 
its  usefulness  is  past,  the  silk  withers 
limply  down.  In  cases  where  no  stigmas 
are  present,  as  for  example  in  many 
cone-bearing  plants,  like  the  Spruces  and 
Pines,  there  is  usually  some  arrangement 
of  smooth  scales  which  guide  the  inci- 
dent pollen  down  to  the  vicinity  of  the 
ovules.  Furthermore,  it  is  obvious  that 
the  efficiency  of  wind  pollination  de- 
pends on  the  greatest  possible  freedom 
of  wind  action  through  the  branches, 
and  therefore  on  absence  of  interference 
by  the  leaves.  This  is  the  reason  why 
so  many  wind-pollinated  flowers  open 


FIG.  109.— Flower  clusters  of  the 
European  Hazel,  a  typical  wind 
pollinated  plant,  showing  the 
great  disproportion  in  bulk  be- 
tween the  male  and  the  female 
flowers,  the  former  being  the 
long  drooping  catkins,  and 
the  latter  the  small  ovoid- 
tufted  structures.  (Copied 
from  Kerner's  Pflanzenleben.) 


308  The  Living  Plant 

in  the  very  early  spring  before  the  leaves  have  appeared,  as 
catkins  for  example  all  do,  and  the  flowers  of  some  Maples; 
while  that  first  feathery  bloom  shown  by  Elms  against  the 
spring  sky  is  caused  by  the  wind-pollinated  flowers,  and  not 
by  the  leaves  as  most  folks  think.  The  same  end  is  attained 
in  a  different  way  in  those  cases  where  the  blossoms  are  borne 
out  at  the  extreme  tips  of  the  branches,  as  in  most  kinds  of 
evergreens,  while  a  still  more  notable  example  is  found  in  the 
Grasses,  which  raise  their  spikes  or  panicles  of  inconspicuous 
greenish  blossoms  high  over  the  leaves,  as  any  meadow  well 
illustrates.  And  a  good  many  other  adaptations  to  wind  pol- 
lination are  found  in  particular  cases.  But  in  general  these 
features, — occurrence  on  trees  in  particular;  light  and  super- 
abundant pollen,  and  therefore  relatively  prominent  male  blos- 
soms; much-branched  stigmas  on  prominently  placed  though 
rather  inconspicuous  female  flowers;  an  early  blossoming  period 
or  an  exposed  blossoming  position — distinguish  the  wind-pollinated 
plants.  And  to  these  characters  may  be  added  another,  of  a 
negative  though  no  less  distinctive  sort,  that  such  flowers  possess 
hardly  any  of  the  features  that  we  commonly  associate  with  the 
name, — no  bright  colors,  aside  from  an  occasional  case  of  the 
early  spring  red,  no  odors,  no  nectar,  no  striking  forms,  no  great 
size.  The  reason  for  their  absence  is  obvious  enough, — such 
features  are  not  needed  in  wind  pollination. 

But  wind  pollination,  widely  used  though  it  is,  becomes  almost 
insignificant  when  compared  with  a  different  method  which  sur- 
passes it  many  fold  in  economy,  efficiency  and  extensiveness  of  use. 
A  great  disadvantage  of  wind  pollination  consists  in  its  waste- 
fulness; for  of  all  the  great  quantities  of  pollen  cast  out  on  the 
winds  from  the  anthers  of  plants,  not  more  than  an  insignificant 
proportion  can  happen  to  fall  on  receptive  stigmas.  One  can 
gather,  indeed,  a  vivid  idea  of  the  wastefulness  of  this  method  from 
the  fact,  which  some  of  my  readers  may  have  seen  for  themselves 
as  I  have,  that  in  northern  countries,  where  wind-pollinated 


Arrangements  for  Securing  Union  of  Sexes        309 

trees,  especially  the  cone-bearing  kinds,  are  particularly  abundant, 
the  little  lakes  of  the  woods  are  covered  in  the  spring  time  with 
pollen  enough  to  make  a  continuous  film  all  over  the  surfaces, 
while  of  course  an  equal  amount  must  fall  on  the  land.  So  plenty 
at  times  is  the  pollen  in  the  air  of  those  countries  that  it  receives 
the  expressive  appellation  of  "sulphur-shower."  Now  pollen, 
composed  as  it  is  almost  wholly  of  the  richest  protoplasmic  ma- 
terial, is  one  of  the  most  difficult  and  expensive  of  substances 
for  plants  to  manufacture;  and  therefore  the  wastefulness  of  wind 
pollination  must  entail  a  great  drain  on  these  plants.  Obviously, 
any  method  which  would  ensure  the  transfer  of  pollen  direct 
from  the  anthers  of  one  plant  to  the  stigmas  of  another  would 
be  greatly  superior  in  both  economy  and  certainty  to  wind  pol- 
lination. Such  a  method,  indeed,  plants  have  developed;  and  it 
consists  in  the  utilization  of  the  locomotive  powers  of  animals, 
especially  insects. 

We  turn,  accordingly,  to  the  study  of  the  cross  pollination  of 
flowers  by  insects.  Obviously  a  first  requisite  of  the  method  is  an 
arrangement  that  will  lead  insects  to  go  directly  from  flower  to 
flower, — a  thing  which  they  will  not  do  unless  induced  by  some 
attraction  or  compulsion.  The  inducement  takes  the  form  of  a 
store  of  nectar, — a  sugary  liquid  both  nutritious  and  palatable 
to  insects,  and  easily  made  by  plants  in  little  superficial  glands. 
These  nectar  glands,  which  often  pour  their  product  into  special 
receptacles  called  nectaries,  and  which  exhibit  a  great  variety  of 
forms  in  different  flowers,  are  of  course  placed  in  close  juxtaposi- 
tion to  the  stamens  and  pistils  (figure  110).  They  constitute  the 
most  fundamental  feature  of  insect-pollinated  flowers;  and  those 
plants  which  possess  them  along  with  stamens  or  pistils  but  no 
other  parts,  for  example  the  Willows  and  some  Maples,  represent 
a  first  stage  in  the  evolution  of  the  insect-pollinated  flower.  But 
a  second  requisite  of  the  method  is  some  arrangement  by  which 
the  position  of  the  inconspicuous  nectar  (and  therefore  of  the 
stamens  and  pistils),  can  be  made  evident  to  the  insects;  and  this 


3io 


The  Living  Plant 


is  accomplished  by  the  provision  of  a  blotch  of  color,  which  is 
formed  and  spread  in  a  special  set  of  leaves  developed  for  the  pur- 
pose,— the  corolla.  This  is  the  reason  for  the  existence  of  color  in 
flowers; — it  is  a  notice  or  signal,  advertising  to  insects  the  position 

of  the  nectar,  which  is  the  real  at- 
traction. Finally,  a  third  requisite 
of  the  method  is  such  a  construction 
of  the  flowers  as  will  make  it  inevi- 
table that  the  insect,  as  it  enters  a 
pollen-ripe  flower  in  the  quest  for  its 
nectar,  shall  receive  on  its  body  a 
supply  of  the  pollen  which  it  will  as 
inevitably  leave  on  the  stigma  in 
entering  an  ovule-ripe  flower.  And 
this  is  the  explanation  of  the  princi- 
pal peculiarities  of  shapes  and  sizes 
in  flowers,  which,  because  insects  are 
most  diverse  in  form  and  habits, 

Fio.   110.-A  flower,  enlarged,  of  the    »«*  themselves  equally  diverse  in  de- 
Rape  with  petals  and  sepals  re-  tajis  of  construction.     Furthermore, 

moved   to  show  the   contiguity  of 

the  nectar  glands  (the  ovoid  struc-    it    is    plain    that    the    reason    for    the 

tures  near  the  base)  to  the  stamens  .  .     ,  •,      .    . ., 

and  pistils.  (Copied  from  Goebei's  separation  of  the  stamens  and  pistils 
into  separate  flowers  in  the  wind- 
pollinated  kinds  does  not  hold  in  those  that  are  pollinated  by 
insects;  for  in  these,  on  the  contrary,  there  are  advantages,  as  to 
economy  of  number  of  blossoms  and  also  of  insect  visits,  in  hav- 
ing both  stamens  and  pistils  associated  in  the  same  flowers.  This, 
accordingly,  is  the  prevailing  condition  in  showy  blossoms. 

Thus  it  is  evident  that  the  most  striking  features  of  the  flowers 
of  the  higher  plants,  including  the  ones  with  which  our  very  con- 
ception of  the  flower  is  most  closely  associated, — the  colored 
corolla,  nectar,  odor,  and  striking  peculiarities  of  shapes,  exist  in 
adaptation  to  cross  pollination  by  insects.  Or,  the  matter  can  be 
stated  in  this  way, — the  flower  is  an  organ  evolved  in  adaptation  to 


Arrangements  for  Securing  Union  of  Sexes        311 

the  advantage  of  the  cooperation  of  two  parent  plants  in  the  produc- 
tion of  offspring. 

In  my  discussion  of  this  subject  I  am  assuming  that  the  reader 
already  has  some  general  knowledge  of  the  relationship  existing 
between  flowers  and  insects.  Surely 
there  is  no  one  whose  attitude  towards 
nature  is  such  as  to  lead  him  to  read 
thus  far  in  this  book,  who  has  not  ob- 
served with  interested  attention  the 
actions  of  insects  among  the  flowers 
in  a  garden;  and  a  little  more  watch- 
ing will  always  reveal  the  same 
things  in  the  flowers  of  field,  road- 
side or  forest.  But  it  may  be  well  if 
I  insert  at  this  point,  in  further  illus- 
tration of  our  subject,  a  description 
of  some  conspicuous  examples  of  ad- 
aptations to  cross  pollination. 

There  grows  commonly  in  Europe, 
and  sparingly  in  this  country  where  it 
has  been  introduced,  a  small  upright 
herbaceous  plant  called  Aristolochia 

FIG.    111.— Flowers    of  Aristolochia 
LlematltlS,       Whose      yellOW       tubular        Clematitis,   just    before    and    just 

blossoms,  an  inch  or  so  long,  stand  S^ftSUX  thTttt 
upright  and  invitingly  open  when  gmpHfied  somewhat  from  Sachs- 
ready  for  fertilization.  It  is  cross 

pollinated  by  small  flies,  which,  bringing  pollen  on  their  bodies 
from  other  flowers,  slip  easily  down  the  tube  through  the 
downward-pointing  hairs  (figure  111).  Then,  working  around 
after  the  nectar  in  the  middle  part  of  the  chamber,  to  which 
they  are  confined  by  other  hairs  in  the  base,  they  leave  their 
pollen  on  the  stigmas  (the  hooked  structures  of  the  figure), 
which  soon  curl  back  out  of  the  way  of  further  pollination.  Im- 
mediately the  hairs  in  the  base  wither  up,  and  the  insects  go  there 


3I2 


The  Living  Plant 


for  the  nectar,  when  the  anthers  open  and  shed  new  pollen  on 
their  bodies.  Then  the  nectar-secretion  ceases,  and  simulta- 
neously the  hairs  in  the  throat,  hitherto  impassable  in  an  up- 
ward direction,  wither  up,  and  the  insect  hies  him  away  with  his 

load  to  another  one  of  the 
flowers.  Finally  the  flower  be- 
comes partially  closed  at  the 
mouth,  as  the  second  figure 
shows,  and  droops  on  its  stalk; 
then  it  is  sought  no  more  by 
insects,  whose  visits  would  ob- 
viously be  useless. 

Another  common  European 
field  plant,  sometimes  seen  in 

FIG.  112.— A  Salvia  flower  (substantially  like  OUr     gardens,      is      that  Mint 

S.   pratensis),  in  general   view  and  in  sec-         11    j     o    7    •               i         •  i_ 

tion,   showing  the   mode   of   cross  pollina-  Called    Salvia    pratCHSlS,  whose 

tion  described  in  the  text.     The  line  on  the  inpVi-lono-       brifrht-hlllP  hori- 

sections  indicates  the  direction  oi  thrust  of  U  OIlg'  U6> 

the  insect's  proboscis.    (Copied,  with  slight  zontally-set,  irregular  flowers 

simplification,  from  the  Chicago  Textbook.) 

possess    stamens    remarkably 

hinged  on  their  stalks  (figure  112).  These  stamens  are  con- 
structed on  the  principle  of  the  lever,  with  the  long  arm 
carrying  the  anthers  up  inside  the  upper  lip,  and  the  short  arm 
resting  down  like  a  valve  over  the  entrance  to  the  nectar  tube. 
The  cross  pollinators  are  bees,  and  when  one  of  these  insects, 
coming  to  a  pollen-ripe  flower,  alights  on  the  lower  lip,  which  is 
suitable  in  size,  form  and  position  for  its  reception,  it  pushes  its 
head  into  the  tube  for  the  nectar  and  thus  forces  back  the  short  arm 
of  the  lever,  which,  swinging  on  the  intermediate  hinges,  brings 
down  its  longer  pollen-laden  end  on  the  back  of  the  bee  in  just  the 
position  where  that  insect  is  struck  by  the  overhanging  stigmas 
as  it  enters  another  flower  that  is  ready  for  fertilization. 

One  of  the  most  wide  spread  of  American  Orchids  is  the  little 
wood-dwelling  Habenaria  orbiculata,  which  sends  up  a  long  loose 
cluster  of  greenish-white  flowers  from  two  glossy  round  leaves 


Arrangements  for  Securing  Union  of  Sexes         313 

spread  flat  on  the  ground.  The  flower,  which  is  shown  greatly 
enlarged  in  the  accompanying  picture  (figure  113),  has  a  struc- 
ture so  remarkable  that  without  elaborate  observational  studies 
no  one  could  ever  imagine  either  the  identity  or  the  use  of  the 
parts.  But  the  strap-shaped  piece  in 
front  is  a  petal;  the  opening  at  its  top 
leads  into  the  greatly-elongated  nectar 
tube  shown  next  behind  it;  the  two  struc- 
tures converging  above  this  opening  are 
the  halves  of  one  anther,  each  of  which 
contains  a  great  many  pollen  grains  tied 
together  into  one  mass  by  threads;  these 
threads  collect  together  into  two  sticky 
discs  shown  as  two  white  oval  structures 
each  side  of  the  opening;  and  the  darker 
space  between  anthers  and  opening  is  the 
stigma.  The  reader  will  readily  recog- 
nize how  different  is  this  construction  from 
that  of  an  ordinary  flower;  and  the  im- 
plication that  the  parts  must  possess  un- 
usual functions  is  correct.  The  cross- 
pollinating  insect  is  a  moth,  with  a 
proboscis  (ordinarily  carried  in  a  pendant  FlG.  113.— Flower,  much  en- 
close coil)  having  a  length  sufficient  to  J^tSJ^IK 
reach  to  the  bottom  of  the  nectar  tube  nated  b-v  the  remarkable 

method     described     in     the 

(figure    114).        It  alights   Upon   the  Strap-        text.     The   hindermost  part, 
•,1,11  i  not  there  mentioned,  is  the 

shaped  petal,  whose  narrowness  compels  ovary  and  8talk.  (Reduced 
its  approach  in  a  very  definite  position,  ^mn^ray>s  structural 
and,  as  it  pushes  far  down  for  the  nectar, 

it  brings  the  two  sides  of  its  head, — its  huge  eyes,  to  be  exact, — 
into  contact  with  the  two  sticky  discs,  which  come  away  with 
their  attached  pollen  as  the  insect  withdraws.  Moths  have 
often  been  caught  with  these  pollen  masses  attached  to  their 
eyes,  which  were  formerly  supposed  to  be  afflicted  by  some  kind 


314  The  Living  Plant 

of  strange  parasite.  Almost  immediately,  —  during  the  flight  of 
the  insect  from  flower  to  flower  in  fact,  —  these  pollen  masses 
droop  on  their  stalks,  and  hang  down  in  such  position  that  when 
the  insect  probes  into  a  new  flower  they  do  not  strike  the  anther 
but  are  pressed  down  directly  on  the  sticky 
stigma,  which  holds  them  tenaciously.  And 
thus  is  cross  pollination  effectively  per- 
formed. 

These  three  cases  have  been  chosen  not 
because  they  are  especially  remarkable,  but 
because  they  illustrate  several  different 
features  of  cross-pollination  methods.  In- 
deed, the  number  of  equally-striking  cases 
is  leSion>  requiring  whole  volumes  for  their 


pollinates  the  Habenaria  adequate  description;  and  many  of  the  ar- 

of  figure  113;  the  pollen  .  . 

masses  are  attached  to  rangements  might  well  stagger  belief  were 

its  eyes.     (Reduced  from      ,  -.    „  ,,  ,     ,  ,  .   .      , 

Gray's  structural  they  not  fully  confirmed  by  the  critical 
studies  of  large  numbers  of  competent  in- 
vestigators. But  while  we  cannot  take  space  to  describe  any  more 
individual  cases,  for  which  the  reader,  if  interested,  may  turn 
to  the  works  described  in  the  footnote,*  we  must  follow  some- 
what farther  a  few  of  the  matters  brought  up  in  the  foregoing 
discussion. 

*  The  principal  works  upon  cross  pollination  likely  to  prove  of  interest  or  use  to 
the  reader  are  the  following:  The  foundation  of  all  is  Sprengel's  book  entitled  (in 
translation,  for  the  work  is  in  German)  Nature's  Secret  displayed  in  the  Construction 
and  Pollination  of  Flowers  (1797),  a  classical  work  a  half  century  ahead  of  its  time, 
and  a  treasury  of  accurate  information  on  its  subject,  though  it  missed,  of  necessity, 
the  central  illuminating  idea  of  the  value  of  cross  as  compared  with  close  pollination. 
Next  in  importance  came  three  of  Darwin's  greatest  books,  The  Various  Contrivances 
by  which  Orchids  are  Fertilized  by  Insects,  The  Effects  of  Cross  and  Self  Fertilization  in 
the  Vegetable  Kingdom,  and  Different  Forms  of  Flowers  on  Plants  of  the  same  Species, 
which  three  works  contain  a  greater  amount  of  new  observation  and  illuminating 
explanation  than  any  others  we  possess.  The  first  general  summary  of  the  entire 
subject  was  Miiller's  Fertilization  of  Flowers,  a  translation  of  a  German  work,  which 
is  admirable  in  all  respects,  and  superseded  only  by  the  cyclopedic  work  by  Knuth, 
Handbook  of  Floral  Pollination,  just  completed,  likewise  a  translation  from  the 


Arrangements  for  Securing  Union  of  Sexes        315 


In  the  first  place  what  is  it  which  prevents  close  pollination  in 
flowers  where  both  sexes  are  present?  Against  this  obvious 
difficulty,  however,  floral  evolution  has  made  ample  provision. 
The  simplest  method  is  a  physiological  one,  viz.,  a  flower  is 
sterile  to  its  own  pollen,  that  is, 
a  given  stigma  will  not  permit  the 
growth  of  its  own  pollen  thereon, 
doubtless  for  some  chemical 
reason;  while  another  phase  of 
the  same  thing  is  the  fact  true  of 
some  plants,  that  if  close  and  cross 
pollen  happen  to  fall  simultane- 
ously on  a  stigma,  the  cross  pollen 
is  the  one  that  grows  fastest  and 
produces  the  fertilization.  But  „ 

ric.    llo. — Dichogamous  flower  of  Clero- 

far    Commoner  is    the    Simple    and        dendron,  on  two  successive  days,  show- 
f      ,1         iv      ,•          j  f  i  ing  the  different    time  of   ripening  of 

perfectly  effective  device  of  mak-      stamens  and  pistils.     (Reduced  from 

ing     the     Stamens     and     pistils    Of        Gray's  Structural  Botany.) 

each  flower  ripen  at  different  tunes,  an  arrangement  called 
dichogamy  (figure  115),  and  found  in  a  good  many  common 
plants.  Again,  close  pollination  is  prevented  by  mechanical 
arrangements,  usually  the  interposition  between  anther  and 
stigma  of  some  specialized  outgrowth,  as  shows  very  well,  for 
example,  in  the  common  Blue  Flag,  or  Iris  (figure  116).  Still 
another  arrangement  is  displayed  by  the  Primroses,  Bluets,  and 
Mayflowers,  which  possess  two  kinds  of  flowers  bearing  stamens 
and  pistils  in  different  positions,  with  corresponding  differences  in 

German.  The  best  general  account  of  the  subject,  admirably  written  and  beauti- 
fully illustrated,  is  contained  in  Kerner's  Natural  History  of  Plants  (another  transla- 
tion from  the  German) ,  while  his  smaller  volume,  Flowers  and  their  Unbidden  Guests, 
is  a  charming  presentation  of  that  subject.  Brief  and  popular  summaries  have  been 
given  by  various  writers,  notably  by  Asa  Gray  in  his  all  too  brief  How  Plants  Behave, 
by  Lubbock,  in  his  Flowers,  Fruits,  and  Leaves,  and  by  W.  H.  Gibson  in  his  Blossom 
Hosts  and  Insect  Guests,  which  is  the  most  readable  of  all  the  works  on  the  subject. 
All  of  these  books  should  be  found  in  the  public  libraries. 


3i6 


The  Living  Plant 


pollen  and  stigmas  (figure  117);  —  a  plan  of  structure  called 
dimorphism.  When  the  suitable  insect  visits  in  succession  several 
flowers  of  the  different  kinds,  it  receives  pollen  on  its  body  from 
the  upper  stamens  in  a  position  to  leave  it  on  the  tall  stigmas,  and 

the  same  for  the  shorter  kinds; 
while  any  accidental  pollination 
of  a  stigma  from  the  same  or  a 
similar  flower  produces  no  effect, 
because  of  the  differences  in  pollen 
and  stigmas  aforementioned. 

But  although  such  elaborate  ar- 
rangements exist  in  adaptation  to 
the  prevention  of  close  pollination, 
in  other  kinds  of  flowers  there  are 
features  which  as  obviously  secure 
it.  Thus,  in  a  great  many  of  the 
simpler  and  regular  kinds  of  flow- 
ers, the  pollen  falls  normally  on 
the  stigmas  of  the  same  flower, 


style  ending  in   a  projecting  shelf  of  and  produces  close  fertilization  in 
which  only  the  upper  surface,  shielded 

from  the  stamen,  is  stigmatic.   (Copied  Case    no    CrOSS    pollen    is    received, 

from  Gray's  Structural  Botany.)  ,,           .       ..                          ...        .. 

though  II   cross  pollination  does 

occur,  then  cross  fertilization  is  effected  instead.  But  a  much 
more  extreme  case  is  found  in  those  flowers  which  never  open 
at  all,  and  in  which  the  pollen-tubes  grow  out  from  the 
anthers  to  the  immediately  contiguous  stigmas,  and  thence 
effect  fertilization  in  the  usual  way  (figure  118).  Such  flowers, 
called  cleistogamous,  lie  close  to  the  ground,  and  are  well  known 
in  Violets  and  some  kinds  of  Oxalis;  but  this  fact  is  conspicuous 
about  them,  that  the  same  plants  in  all  cases  possess  also  the 
ordinary  showy  kinds  of  blossoms  cross  pollinated  by  insects. 
Obviously,  therefore,  cleistogamous  blossoms,  like  the  cases  of 
close  pollination  earlier  mentioned,  represent  a  method  of  en- 
suring close  fertilization  in  case  a  cross  should  happen  to  fail, 


Arrangements  for  Securing  Union  of  Sexes         317 


117. — Dimorphous  flowers, 
enlarged,  of  Partridge  Berry, 
further  explained  in  the  text. 
(Reduced  from  Gray's  Struc- 
tural Botany). 


on  the  principle  that  although  cross  fertilization  is  better  than 
close,  a  close  fertilization  is  better  than  none.  And  for  this 
principle  there  is  much  other  evidence.  In  all  of  these  cases, 
however,  at  least  an  occasional  cross  fertilization  must  be  ef- 
fected; and  there  is  good  reason  to  be- 
lieve that  while  many  kinds  of  plants 
can  endure  close  fertilization  for  a  con- 
siderable time,  they  must  have  an  oc- 
casional cross  in  order 'to  retain  their 
full  vigor. 

But  we  must  turn  for  a  moment  to 
view  cross  pollination  from  the  side  of 
the  insect.  Our  discussion  thus  far  may 
have  seemed  to  imply  that  insects  Fl 
exist  in  certain  sizes,  forms,  and  habits, 
fixed  by  other  considerations,  and  that 
the  adaptations  between  them  and 

flowers  have  been  wholly  effected  by  modifications  of  the  flow- 
ers. This,  however,  is  not  correct,  for  there  is  every  evidence 
that  in  the  course  of  evolution,  insects  have  become  adapted 
to  flowers  as  well  as  flowers  to  insects,  as  indeed  we  might  expect 
from  the  fact  that  while  it  is  an  advantage  for  flowers  to  have 
their  pollen  carried  by  insects,  it  is  an  advantage  to  insects  to  be 
able  to  obtain  their  food  from  the  flowers.  There  are,  of  course, 
many  kinds  of  insects  which  never  visit  flowers  at  all;  and  it  is 
only  the  kinds  which  are  nectarivorous,  so  to  speak,  that  plants 
have  been  able  to  provide  an  attraction  for,  and  only  these  kinds 
show  adaptations  to  flowers. 

For  the  success  of  cross  pollination  of  flowers  by  insects,  it  is 
obviously  essential  that  the  insects  shall  habitually  visit  plant 
after  plant  of  the  same  kind,  rather  than  first  one  kind  of  plant 
then  another,  which  happen  to  blossom  together.  For  no  result 
follows  a  cross  pollination  between  different  kinds.  Observation 
always  shows  that  in  fact  insects  do  as  a  rule  visit  the  same  kinds 


318  The  Living  Plant 

of  plants  successively,  as  anyone  can  see  for  himself  in  a  garden; 
while  experiment  indicates  that  they  are  primarily  guided  by 
color,  which,  probably,  in  their  equivalents  for  minds,  becomes 
associated  with  flowers  in  which  the  nectar  is  ready.  This  process 


FIG.  118. — A  plant  of  the  common  Blue  Violet,  displaying  the  contrast  between  the 
familiar  showy  flowers  and  the  cleistogamous  kind,  which  are  the  bud-like  struc- 
tures on  the  recurved  lower  stems.  At  the  right  is  a  cleistogamous  flower  in  section, 
showing  the  contiguity  of  anthers  and  stigma.  (Copied  from  Atkinson's  Textbooks.) 

is  greatly  aided  in  nature  by  a  correlative  peculiarity  in  the  plants 
themselves, — namely,  that  different  kinds  of  flowers  which 
blossom  together  at  the  same  tune  are  usually  strongly  contrasted 
in  color,  as  any  meadow,  or  brookside,  or  autumn  roadside  il- 
lustrates. It  is  true  that  the  insects  do  not  visit  only  one  flower 


Arrangements  for  Securing  Union  of  Sexes        319 

on  a  plant  and  then  visit  only  one  on  another,  as  would  be  theo- 
retically the  best  of  arrangements;  for  on  the  one  hand  that  were 
too  difficult  a  thing  for  the  plant  to  be  able  to  induce  the  insect 
to  do,  and  on  the  other  it  is  needless.  What  happens  in  reality  is 
this,  that  an  insect  in  visiting  a  plant  usually  goes  successively  to 
all  the  flowers  that  are  open,  and  thus  becomes  thoroughly  dusted 
all  over  by  a  mixture  of  pollen,  which  is  ample  in  quantity  to 
allow  some  for  each  stigma  of  all  of  the  flowers  on  the  next  plant 
that  it  visits.  Of  course  there  is  mixture  of  pollen,  and  a  great 
deal  of  pollination  between  different  flowers  on  the  same  plant; 
but  the  method  makes  probable  the  presence  of  some  cross  pollen 
on  each  stigma,  when  the  selective  power  of  the  stigma  for  cross 
pollen,  already  mentioned,  ensures  cross  fertilization.  And  the 
matter  is  aided  a  good  deal  by  a  peculiarity  of  blossoming  which 
practically  all  plants  show,  that  no  large  number  of  flowers  are 
open  at  one  tune  in  the  same  cluster, — no  more,  one  may  say, 
than  as  many  as  an  insect  can  pollinate  by  the  quantity  of  pollen 
it  can  carry  on  its  body  from  a  previously-visited  plant.  Of 
course  none  of  these  arrangements  are  exact  in  their  working,  but 
are  general,  or  average,  or  clumsy,  with  many  individual  failures. 
But  on  the  whole  they  suffice. 

That  insects  find  flowers  chiefly  through  the  colors  seems  un- 
doubted, but  there  is  more  in  the  subject  than  appears  at  first 
sight.  The  chief  essential  of  floral  color,  from  this  point  of  view, 
is  conspicuousness,  which  of  course  involves  contrast  with  the 
background;  and  as  this  is  commonly  green,  therefore  white  and 
yellow  and  red  are  the  commonest  of  floral  colors,  especially  in 
flowers  that  nestle  among  foliage.  The  less  contrasted  blue  is 
rather  more  common  in  flowers  that  stand  out  by  themselves, 
whether  singly  or  in  long  terminal  clusters.  Furthermore,  it  is 
true  that  some  kinds  or  groups  of  insects  show  preference  for 
certain  floral  colors;  and,  correlatively,  the  flowers  having  such 
colors  are  prevailingly  of  a  size  and  construction  better  fitted  to 
the  visits  of  those  insects  than  of  others.  Thus,  most  small  and 


320  The  Living  Plant 

regular  open  flowers  are  yellow  or  white,  and  visited  by  a  great 
variety  of  small  insects,  especially  flies.  Blue  flowers,  however, 
are  visited  mostly  by  bees,  and,  as  the  Larkspur  and  Monkshood 
well  illustrate,  possess  in  general  a  position  of  nectar,  a  compul- 
sory mode  of  access  thereto,  and  an  arrangement  of  stamens  and 
stigmas  such  that  bees  can  best  of  all  insects  get  the  nectar  and 
most  surely  carry  the  pollen.  Red  flowers,  such  as  the  Pinks,  are 
oftenest  visited  by  Butterflies,  whose  probosces  are  long  enough 
to  reach  to  the  bpttom  of  their  slender  tubes  for  the  nectar  which 
is  there  inaccessible  to  the  very  much  shorter  probosces  of  Bees. 
Again,  white  flowers,  in  highly  specialized  kinds  like  the  Orchids, 
are  preferred  by  Moths,  which  are  indeed  the  only  insects  possess- 
ing probosces  of  a  length  sufficient  to  reach  to  the  bottoms  of  the 
unusually  long  nectariferous  tubes.  The  reason  of  course  why 
the  insects  prefer  the  respective  colors  is  because  these  have 
come  to  be  associated  with  a  construction  of  flower  from  which 
they  can  easily  draw  nectar,  while  that  nectar  is  pretty  sure  to  be 
present  because  other  kinds  of  insects  are  largely  excluded.  These 
relations,  as  before,  are  not  precise  in  detail,  but  operate  as  a 
general  principle;  and,  as  a  general  principle,  also,  it  is  true  that 
insects,  floral  colors,  and  floral  structure  have  evolved  together  in 
harmonious  correlation. 

While  considering  this  subject  of  floral  colors,  I  may  here  add 
a  number  of  miscellaneous  matters  of  particular  interest.  Thus, 
as  to  white  color,  it  is  found  to  distinguish  most  flowers  that  bloom 
in  the  dusk  of  the  evening,  that  being  of  course  the  one  color 
which  is  most  conspicuous  in  darkness;  and  such  flowers  commonly 
exhibit  the  very  long  nectar-tubes  and  other  constructional 
features  adapting  them  to  the  visits  of  moths,  which  are  chiefly 
night-flying  in  habit.  This  is  the  explanation  of  the  peculiarities 
of  the  Night-blooming  Cereus,  Nicotiana,  and  some  Jessamines. 
Quite  a  different  aspect  of  floral  conspicuousness  is  involved  in 
the  brilliant  coloration  of  flowers  that  grow  in  rather  inhospitable 
places,  such  as  Arctic  shores,  Alpine  heights,  and  desert  wastes. 


Arrangements  for  Securing  Union  of  Sexes         321 

Alpine  plants  in  particular  are  famous  for  their  beautiful  colora- 
tion. An  explanation  thereof  has  been  found  in  adaptation  to  the 
comparative  scarcity  of  insects  in  these  places,  the  extra  brilliancy 
representing  the  extra  difficulty  of  ensuring  their  visits.  Again, 
a  good  many  flowers  exhibit  a  considerable  variegation  of  color, 
consisting  chiefly  of  definite  spots  or  lines  quite  different  in  hue 
from  the  ground  color  of  the  flower  as  a  whole,  as  Forget-me-nots 
and  Nasturtiums  well  illustrate.  But  these  markings  are  found 
always  to  have  one  feature  in  common, — that  they  indicate  the 
position  of  the  nectar.  The  floral  color  as  a  whole  brings  the 
insect  to  the  flower  from  a  distance,  and  these  markings  then  show 
it  the  place  to  probe  for  the  nectar, — which  of  course  brings  it 
into  the  position  where  it  can  best  leave  its  pollen  and  receive  an 
additional  supply.  Again,  the  effectiveness  of  color  is  obviously 
increased  by  massing,  which  explains  the  value  of  clusters  of 
flowers,  especially  for  kinds  that  are  small.  Finally,  as  to  this 
matter  of  color,  we  need  note  but  one  more  peculiarity.  Some 
kinds  of  flowers,  though  none  that  are  very  familiar,  change  color 
immediately  after  fertilization;  and  it  is  claimed  that  such  flowers 
are  no  more  entered  by  insects,  whose  visits  would  obviously  be 
useless  to  both  the  flowers  and  themselves.  The  same  end  is  here 
attained,  though  by  a  different  method,  as  in  the  case  of  the 
drooping  flowers  of  the  Aristolochia  already  described.  The  ad- 
vantage to  the  species  as  a  whole  of  preventing  useless  visits  of 
insects,  and  thereby  conserving  then-  services  for  flowers  which 
still  need  them,  is  sufficiently  obvious. 

As  an  advertisement  to  insects  of  the  position  of  the  flower, 
color  often  is  aided,  and  sometimes  replaced,  by  odor.  It  has 
even  been  claimed  in  late  years  that  insects  are  guided  to  flowers 
much  more  by  odors  than  colors,  many  of  such  odors  being  hardly, 
or  not  at  all,  perceptible  by  us;  but  the  evidence  on  this  point  has 
not  yet  won  acceptance.  However,  there  is  no  question  at  all  as 
to  the  assistance  rendered  by  odor  to  color  in  those  cases  where 
color  alone  cannot  be  made  sufficiently  conspicuous.  This  is  true 


322  The  Living  Plant 

especially  of  night-blooming  flowers,  in  which  the  association  of 
sweet  odor  and  white  color  is  very  common.  This  same  aid  of 
odor  to  color  is  found  in  those  flowers  which  bloom  in  very  in- 
conspicuous positions,  such  as  close  to  the  ground,  or  among 
leaves  in  the  shade,  as  the  Mayflower  illustrates;  and  in  general 
odoriferous  flowers  that  are  not  night-blooming  are  the  shy  little 
kinds  of  the  woods.  Odor  also  aids  color,  or  acts  as  a  substitute 
in  some  flowers  which  have  not  attained  to  a  corolla,  or  have  lost  it, 
as  in  some  Willows  and  Maples.  On  the  other  hand,  flowers  that 
grow  in  exposed  places,  and  display  an  abundance  of  color,  very 
rarely  possess  any  odor,  as  the  tall  kinds  of  the  meadows,  the 
river-banks,  the  autumn  roadsides  and  the  prairies  all  illustrate, — 
the  absence  of  sweet  flowers  from  the  prairies  in  particular  being 
matter  of  common  knowledge  and  frequent  comment.  And 
finally,  as  to  odor,  we  need  note  but  one  more  point,  that  while 
most  floral  odors  happen  to  be  pleasing  to  us,  there  are  some  that 
are  not,  as  in  case  of  the  Skunk  Cabbage  and  a  good  many  others 
of  that  family.  But  such  odors  have  their  lovers  among  insects 
to  which  they  are  doubtless  more  sweet  than  all  of  the  spices  of 
Araby.  Indeed,  it  is  only  a  fortunate  accident  that  any  of  the 
odors  of  plants  give  us  pleasure  at  all;  for  in  their  evolution  our 
tastes  in  the  matter  were  not  in  the  least  consulted. 

Color  and  odor  suggest  nectar,  which  is  the  real  attraction  to 
insects  in  the  great  majority  of  flowers.  It  can  usually  be  seen 
very  easily  at  the  bottoms  of  floral  tubes  where  it  lies  as  a  clear 
watery  liquid ;  and  sometimes  in  special  receptacles  of  more  open 
flowers  it  stands  out  in  great  glistening  drops,  as  conspicuously 
illustrated  by  the  Crown  Imperial.  However,  a  good  many 
flowers  are  without  it  entirely,  in  which  case  the  attraction  is 
pollen,  then  produced  in  unusual  abundance;  for  some  insects 
prefer  pollen  to  nectar,  making  use  of  it  not  only  for  food,  but 
also  for  building  their  honeycomb  cells.  And  if  the  reader  should 
ask  me  why  some  flowers  use  nectar  while  others  use  pollen  as 
their  means  of  attraction,  I  agree  that  I  will  tell  him  when  he  has 


Arrangements  for  Securing  Union  of  Sexes        323 

told  me  why  one  man  is  a  carpenter  and  another  a  farmer;  or  why 
the  Latin  races  are  artistic  while  the  Teutonic  are  practical;  or 
why  the  Germans  are  the  best  scientific  investigators  in  all  the 
world. 

The  symmetry  of  my  subject  would  seem  to  demand  that  I  add 
to  these  paragraphs  on  color,  odor,  and  nectar,  another  devoted 
to  the  mechanical  arrangements  in  flowers  in  relation  to  cross 
pollination.  But  I  despair  of  giving  any  adequate  idea  of  this 
subject  in  the  space  that  remains  at  my  command,  and  it  must 
suffice  to  say  that  such  arrangements  are  both  remarkable  and 
innumerable,  involving  not  only  the  most  extreme  modifications 
in  all  of  the  parts,  but  such  special  features  as  sensitively-bend- 
ing stamens  (in  the  Barberries),  closing  stigmas  (perhaps,  in 
Mimulus),  springing  stamens  (as  in  Mountain  Laurel),  explosive 
stamens  (as  in  Mallows),  forcibly-projected  pollen-masses  (as  in 
some  Orchids),  and  others  as  striking,  which  the  reader  may 
follow  as  far  as  he  pleases  through  the  many  good  books  devoted 
to  the  subject. 

It  is  doubtless  sufficiently  obvious  why  insects  are  the  animals 
most  used  for  cross  pollination  by  plants,  for  their  small  size, 
active  flight,  and  especially  their  nectarivorous  habits,  make  them 
especially  available  for  this  purpose.  But  it  must  at  the  same 
time  be  remembered  that  those  very  features  have  doubtless  in 
large  part  been  evolutionarily  acquired  in  conjunction  with  the 
corresponding  features  of  the  flowers.  Insects,  however,  are  not 
the  only  animals  thus  utilized,  for  certain  nectarivorous  birds,  of 
which  the  Humming-bird  is  the  most  familiar  example,  cross 
pollinate  flowers  in  quite  the  same  manner  as  the  insects.  Every- 
body has  seen  in  our  own  gardens  the  Trumpet-creepers  and 
Nasturtiums  and  Scarlet  Salvias  visited  by  Humming-birds. 
There  are  plenty  of  tropical  flowers,  displaying  for  the  most  part 
large  tubular  corollas,  abundant  nectar,  and  scarlet  colors,  which 
have  a  form,  size,  and  shape  well  suited  to  the  flying-habits  of 
those  birds.  Among  other  animals  that  effect  cross  pollination 


324  The  Living  Plant 

are  the  Snails,  which  are  said  to  visit  some  low-growing  flower- 
spikes  of  tropical  plants  for  the  soft  tissue  that  grows  abundantly 
among  the  blossoms;  and  thus  they  transfer  pollen  from  one 
plant  to  another.  But  the  other  groups  of  animals  are  unavailable, 
for  obvious  reasons  of  habit,  size,  structure  and  the  like. 

As  an  earlier  chapter  (on  Protection)  has  indicated  already, 
plants  are  obliged  not  only  to  develop  structures  in  adaptation  to 
the  performance  of  their  functions,  but  also  to  protect  them  when 
made  from  hostile  external  forces  which  would  work  their  destruc- 
tion. This  is  all  very  true  of  the  highly  complicated  and  greatly 
exposed  flowers.  A  certain  protection  against  hostile  wreather 
conditions  is  attained  by  a  control  over  the  time  of  blossoming, 
which  occurs  in  most  plants  only  at  times  and  seasons  when  the 
conditions  are  favorable  for  cross  pollination,  the  blossoms  open- 
ing in  fine  weather  when  insects  are  about,  but  not  during  rain- 
storms, when  they  remain  under  shelter.  One  of  the  greatest 
dangers  to  which  the  cross-pollinating  mechanisms  are  liable  is 
the  influence  of  rain  on  the  pollen,  for  water  is  absorbed  os- 
motically  by  many  kinds  to  a  degree  which  causes  the  bursting  and 
destruction  of  the  grains.  Accordingly,  in  flowers  many  arrange- 
ments exist  in  adaptation  to  protection  of  pollen  from  rain,  aside 
from  the  great  one  already  mentioned, — the  failure  of  blossoms  to 
open  in  stormy  weather.  Thus,  in  a  great  many  blossoms  the 
anthers  are  safely  sheltered  under  an  overhanging  upper  lip,  as  in 
most  irregular  flowers,  like  the  Mints,  Monkshood,  and  others  of 
horizontal  position,  while  in  some  kinds  they  are  guarded  by 
bands  of  unwrettable  hairs.  Again,  some  kinds  of  flowers  close  in 
threatening  weather,  while  others,  arranged  in  flat-topped  clusters, 
turn  upside  down  in  a  rain-storm,  presenting  an  aspect  which 
leads  most  people  to  imagine  that  they  have  been  beaten  over  by 
force  of  the  rainfall. 

But  an  especial  protective  need  of  flowers  is  against  insects  that 
are  not  adapted  to  cross  pollinate  them,  and  which  would  remove 
the  nectar  without  rendering  any  service  in  return, — against 


Arrangements  for  Securing  Union  of  Sexes        325 

"unbidden  guests,"  as  Kerner  so  happily  called  them.  In  one 
instance,  at  least,  plants  seem  quite  helpless  against  such  an 
attack,  for  Bees  often  puncture  the  nectariferous  spurs  of  Colum- 
bines and  Larkspurs  in  our  gardens  without  entering  the  flower  at 
all;  but  this  is  exceptional,  and 
presumably  a  recently-acquired 
habit  of  those  insects.  A  partial 
protection  against  unbidden  guests 
is  secured  by  the  adaptation  of 
floral  to  insect  shapes  already  de- 
scribed, in  correlation  with  which 
most  insects  visit  only  the  flowers 
to  which  they  are  fitted,  leaving 
the  others  alone.  But  there  is  one 

InnH    nf    in«ppf      whn«A    «ma11     01-70    FlG<  119-— Interior  of  a  flower  of  Cobaea 
Kind    01    insect,    \\nOSe    Small    Size        scandenSt  showing  the  masses  of  hairs 

and  other  characteristics  make  it      commonly  believed  to  protect  the 

nectar  from  insects  unadapted  to  ef- 

USeleSS  as    a    CrOSS   pollinator,    but        feet  cross  pollination.     (Copied  from 
i-    i     •          i    •*  ,  •  Kerner's  Pflanzenleben.) 

which  is  at  the  same  time  a  par- 
ticularly pertinacious  nectar  lover,  and  that  is  the  Ant,  against 
which,  accordingly,  especial  protection  is  needed.  A  number 
of  adaptions  preventive  of  its  access  to  nectar  appear  to 
exist.  Possibly  the  extra-floral  nectaries  earlier  described 
(page  212),  may  provide  a  bait  to  keep  these  insects  from 
the  flowers.  Furthermore  this  is  probably  the  explanation  of 
the  closure  of  the  throats  of  flowers,  best  exemplified  in  the 
Snapdragon,  in  a  way  to  open  by  the  pressure  of  a  large  insect's 
weight  or  strength  but  not  to  the  small  body  of  an  ant;  while  the 
rings  of  scales  or  hairs  in  the  throat  or  somewhere  in  the  tube  of 
the  flower  (figure  119),  or  sticky  glands  all  over  the  outside  of  the 
calyx  or  neighboring  parts  (figure  120),  have  probably  the  same 
explanation,  as  have  a  number  of  other  arrangements  of  minor 
account  described  by  Kerner  in  his  charming  book  devoted  to  the 
subject. 

It  is  thus  plain  that  flowers,  like  other  parts  of  the  plant,  are 


326  The  Living  Plant 

never  the  expression  of  adaptation  to  some  single  function  alone, 
but  represent  a  resultant  or  compromise  between  adaptation  to 
some  leading  function  and  adaptation  to  a  number  of  minor  ones, 
—  the  whole  being  further  modified  by  the  influence  of  a  quantity 
of  other  factors,  —  mechanical,  incidental  and 
hereditary. 

In  this  discussion  of  cross  pollination  and  the 
flower,  which  involves  some  of  the  most  com- 
plicated and  efficient  of  all  known  adapta- 
tions, the  reader  must  have  noticed  how  closely 
the  mode  of  presentation  of  ideas,  and  even  the 

FIG.  120.—  A  flower  of    .  A.      .     .  ,  ,        ...      ,. 

the  Linruea.  or  Twin-  language  that  is  used,  correspond  with  those 


which  are  commonly  employed  in  describing 
the  base  of  the  flower,  some  great  product  of  human  activity,  —  the 

supposed  to  protect  .         . 

it  from   access  of  organization  of  society,  government,  or  a  great 

creeping  insects.  ,  .       ,     ,,  .  ,.      .,  .  ... 

business.  And  this  peculiarity  of  exposition 
is  not  confined  to  the  present  writer  alone,  but  seems  una- 
voidable by  any  author  who  seeks  to  make  the  subject  under- 
stood. It  arises  of  course  in  some  part  from  our  common  custom 
of  personifying  nature  for  purposes  of  convenient,  economical, 
and  vivid  expression,  but  in  much  larger  part,  I  am  convinced, 
from  a  more  or  less  unconscious  recognition  of  the  fact  that  there 
is  an  actual  correspondence,  or  even  an  identity,  between  man's 
way  of  effecting  results,  and  nature's.  It  is  not  that  nature  thinks 
things  out  as  a  man  does,  but  that  mind  in  a  man  works  things  out 
as  nature  does.  This  must  be  true,  indeed,  on  theoretical  grounds, 
else  we  must  maintain  that  the  mind  of  man  is  not  an  evolution 
with  its  roots  in  the  rest  of  nature,  but  a  special  creation  of  its 
own  separate  kind;  and  against  such  a  conception  is  arrayed  all 
of  the  natural  knowledge  we  possess.  In  all  exposition,  therefore, 
it  is,  as  I  think,  scientifically  correct  as  well  as  practically  con- 
venient, to  personify  nature. 


CHAPTER  XIII 

THE  WAYS  IN  WHICH  PLANTS  INCREASE  IN  SIZE  AND 
FORM  THEIR  VARIOUS  PARTS 

Growth;  physiological 

F  all  the  physiological  processes  of  plants,  the  one  that 
possesses  the  greatest  interest  for  most  people  is 
Growth.  It  is  really  a  remarkable  phenomenon,  no 
matter  how  one  views  it, — whether  in  the  unfolding 
and  perfecting  of  some  favorite  flower,  foliage,  or  fruit:  in  the 
development  of  a  single  microscopical  egg-cell  through  embryo 
seedling  and  sapling  to  a  mammoth  tree :  or  in  the  seasonal  proces- 
sion of  vegetation  from  the  dormance  of  winter  through  the  un- 
folding of  spring,  the  maturity  of  summer,  and  the  fruition  of 
autumn.  I  take  it  the  reader  does  not  share  in  the  mischievous 
fallacy  that  to  know  the  causes  of  things  is  to  lessen  one's  enjoy- 
ment of  them,  and  I  shall  try  to  describe  the  way  in  which  these 
various  results  come  about. 

At  first  sight  the  phenomena  of  growth  seem  too  heterogeneous 
for  analysis,  but,  like  many  another  complication,  they  separate 
out  in  their  true  proportions  under  persistent  investigation.  And 
the  first  far-reaching  fact  which  stands  out  is  this,  that  growth 
consists  of  three  operations,  which,  often  in  progress  together,  are 
really  distinct  in  their  nature  and  can  proceed  quite  apart  from 
one  another.  These  are, — formation  of  new  parts,  or  development, 
increase  in  size,  or  enlargement,  and  ripening  for  the  final  func- 
tion, or  maturation.  The  distinction  comes  out  very  well  in  the 
case  of  the  spring  vegetation.  Everybody  knows  that  the  flowers 

327 


328  The  Living  Plant 

and  leaves  which  burst  forth  at  the  first  coming  of  spring  were 
formed,  or  developed,  the  season  before,  and  existed  over  winter 
tucked  away  very  snugly  in  well-covered  buds.  In  a  Horse 
Chestnut  bud,  for  example,  one  can  recognize  by  dissection,  at 
any  tune  in  winter,  the  flowers  and  leaves  which  are  to  come  out 
the  next  spring;  and  the  same  thing  can  be  seen  even  more  clearly 
in  sections  made  through  flowering  bulbs  (Hyacinth,  Tulip, 
Crocus) .  Seeds  with  their  embryos  act  the  same  way.  In  all  of 
these  cases  the  formation  or  development  of  the  parts  takes  place 
in  early  fall ;  the  principal  part  of  their  increase  in  size,  or  actual 
growth,  occurs  the  next  spring;  while  the  full  ripening  of  parts, 
such  as  leaves,  for  the  complete  performance  of  their  functions, 
follows  in  summer.  This  shows  how  distinct  the  three  phases  of 
growth  can  be.  Accordingly  we  can  best  consider  them  separately, 
and  for  practical  reasons  may  begin  with  the  most  familiar, — 
increase  in  size,  or  enlargement. 

Plants,  unlike  animals,  grow  by  repetition  of  similar  parts, — 
new  leaves,  stems,  roots,  flowers,  and  fruits  being  formed  in  an 
endless  succession.  We  shall  therefore  first  direct  our  attention 
to  the  growth  of  these  individual  parts,  of  which  the  stems  grow 
the  fastest  and  are  easiest  to  study.  Anyone  can  determine  the 
rate  of  growth  of  stems  in  a  general  way  by  making  frequent 
measurement  with  rulers  placed  alongside  the  plant.  For  scien- 
tific purposes,  of  course,  very  exact  ways  have  been  devised,  not 
only  for  measuring  growth,  but  even  for  compelling  a  growing 
stem  to  register  its  own  growth  upon  paper.  One  of  the  best  of 
such  instruments  is  shown  in  our  accompanying  figure  (figure  121), 
and  the  reader  may  confide  in  my  judgment  of  its  merits,  because 
I  am  myself  the  inventor.  To  the  extreme  tip  of  the  stem  is 
attached  a  thread,  which  is  then  run  over  a  small  wheel,  as  shown 
in  the  figure,  and  there  fastened.  Around  the  rim  of  the  larger 
wheel,  which  is  one  piece  with  the  smaller,  runs  another  thread 
which  passes  over  a  small  pulley-wheel  and  carries  a  pen  against 
a  paper-covered  cylinder.  The  weight  of  this  pen  just  suffices  to 


Ways  in  Which  Plants  Increase  in  Size 


329 


turn  the  wheels  and  keep  the  threads  taut;  and  therefore,  as  the 
plant  grows,  the  pen  descends,  making  its  mark  upon  the  paper. 
The  descent  of  the  pen,  however,  is  obviously  faster  than  the 
growth  of  the  plant  in  just  the  proportion  that  the  greater  wheel  is 
larger  than  the  smaller,  this  arrangement  of  the  wheels  being 


FIG.  121. — An  auxograph,  or  recording  growth  measurer,  in  action.  Its  construction  is  ex- 
plained in  the  text.  Unfortunately  the  record,  in  the  form  of  a  spiral  line  on  the 
cylinder,  does  not  show  in  the  picture. 

adopted  in  order  to  space  out  the  growth  record  enough  for  clear 
visibility.  The  cylinder,  however,  is  revolved  continuously  by 
clockwork,  making  a  complete  turn  once  every  hour;  and  there- 
fore the  descending  pen  traces  not  a  straight,  but  a  spiral,  line, 
which  every  hour  crosses  a  vertical  line  ruled  on  the  paper,  mark- 


33° 


The  Living  Plant 


ing  off  thereon  the  precise  growth,  magnified  of 
course  proportionally  throughout.  The  papers  can 
then  be  removed  from  the  cylinders  and  joined  end 
to  end  in  a  continuous  roll,  or  else  a  flat  band.  Thus 
is  a  plant  made  to  write  its  own  record  of  growth  in 
a  way  convenient  for  scientific  use.  Such  a  record, 
obtained  by  one  of  my  own  students,  and  showing  the 
growth  of  the  flower-stalk  of  a  Grape  Hyacinth  from 
its  first  appearance  above  ground  to  the  completion 
of  flowering,  is  shown,  greatly  reduced  of  course,  in 
the  accompanying  illustration  (figure  122).  And 
by  suitable  modifications  of  the  same  auxograph  (for 
so  it  is  called  because  it  is  a  growth  writer),  the 
growth  of  roots,  leaves  and  other  parts  can  likewise 
be  registered. 

A  growth  record  like  that  of  our  figure  is  very  ex- 
pressive, but  the  facts  can  be  brought  out  still  better 
in  the  form  of  a  graph  like  that  which  already  has 
been  used  and  described  under  Transpiration;  and 
such  a  graph  is  presented  in  our  figure  123.  The 
base  line  is  laid  off  in  divisions  of  time,  each  space 
representing  one  hour,  while  the  vertical  lines  are 
marked  off  with  the  number  of  millimeters  of  growth 
(magnified)  per  one-hour  period,  these  marks  being 
joined  by  straight  lines  in  the  usual  way.  In  the 
resulting  polygon,  as  the  reader  can  see,  the  rise  and 
fall  of  the  lines  corresponds  to  the  rise  and  fall  in  the 
rate  of  growth.  The  reader  must  remember  that  such 
a  graph  represents  the  rate  of  the  growth,  not  its 
amount,  which  fact  explains  the  feature,  puzzling 
to  some  people,  that  a  growth  graph  can  fall  as  well 
FIG.  122.— Pho-  as  rise. 

tograph, re- 
duced to  one-tenth  the  true  size,  of  the  record  papers  taken  from  the  cylinder  of  the 
auxograph  (of  figure  121)  during  the  growth  of  a  flower-stalk  of  Grape  Hyacinth. 
The  heavier  cross  lines  indicate  noon  of  each  day. 


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332  The  Living  Plant 

When,  now,  we  inspect  this  graph  somewhat  closely  we  find 
its  most  remarkable  feature  to  consist  in  its  great  irregularities; 
and  the  same  thing  appears  in  any  others,  from  whatsoever  source 
they  are  taken.  In  other  words,  the  growth  of  plant-structures  is 
extremely  irregular  in  rate.  It  will  not  take  the  reader  very  long 
to  ascribe  the  irregularities  to  the  real  cause  of  the  most  of  them, 
namely, — variations  in  the  external  conditions  of  temperature, 
light,  moisture  and  so  forth.  In  order  to  determine  the  precise 
effect  of  each  of  these  conditions,  it  is  only  necessary  to  plot  the 
simultaneous  graphs  of  temperature,  moisture,  and  light,  ob- 
tained as  already  described  under  Transpiration,  upon  the  same 
sheet  with  the  growth  graph;  and  this  has  been  done  in  the  ex- 
ample presented  above  (figure  123).  This  subject  of  the  effect  of 
external  conditions  upon  growth  is,  however,  so  important,  that  it 
must  be  considered  somewhat  farther. 

First,  as  to  the  effects  of  temperature  upon  growth.  Every- 
body knows,  in  a  general  way,  that  plants  grow  faster  in  warm 
weather  and  slower  in  cold ;  and  in  the  early  spring  we  see  ample 
illustration  thereof  in  the  way  the  grass  comes  up  fastest  in  the 
warmest  corners,  or  in  places  where  warm  pipes,  such  as  sewers 
from  houses,  cross  lawns, — marking  their  courses  by  the  early 
greenness  above  them.  In  our  graphs  the  reader  can  see  how 
closely  the  rise  and  fall  in  growth  rate  is  connected  with  the  rise 
and  fall  of  the  temperature.  The  same  thing  is  shown,  and  very 
much  clearer,  by  an  instrument,  devised  for  the  purpose,  and 
shown  in  our  figure  (figure  124).  It  must  suffice  to  say  that  by 
its  aid  one  can  determine  in  a  continuous  band  of  soil  the  lowest 
temperature  at  which  a  plant  can  be  made  to  grow  (the  minimum}, 
the  temperature  at  which  it  grows  its  very  best  (the  optimum), 
and  that  above  which  it  will  not  grow  at  all  (the  maximum). 
Between  the  minimum  and  maximum,  the  tips  of  the  growing 
plants  plot,  as  it  were,  their  own  curve  of  the  relation  of  growth 
to  temperature,  culminating  at  the  optimum,  as  our  picture  well 
shows. 


Ways  in  Which  Plants  Increase  in  Size  333 

These  three  cardinal  points  vary  much  with  different  plants, 
ranging  lower  in  those  of  cold  regions  and  higher  in  those  of 
the  tropics;  and  plants  can  thrive  only  in  climates  where  the 
range  of  usual  temperature  corresponds  somewhat  closely  with 
their  cardinal  points.  This  will  explain  why  the  Orange  will  not 
grow  if  planted  in  Canada,  or  Barley  and  Rye  if  taken  to  Florida. 
In  plants  of  our  own  climates  these  points  approximate  on 
the  average  to  5°-30°-40°  Centigrade  respectively  (or  40°-85°- 
100°  Fahrenheit),  which  means  that  most  of  our  plants  do  not 
grow  appreciably  below  40°;  they  grow  best  at  about  85°;  and 


FIG.  124. — A  graphic  illustration  of  the  relation  of  growth  to  temperature.  The  copper 
trough  is  heated  from  one  end  (the  left),  and  chilled  from  the  other,  with  the  result 
that  the  temperatures  grade  evenly  between. 

hardly  grow  at  all  above  100°.  This  will  explain  why  it  is  that 
when  the  temperature  of  our  fields  rises  higher  than  100°  in  the 
sun,  the  extra  heat  is  no  aid  to  plant  growth,  being  rather  a 
hindrance  thereto.  The  same  thing  would  happen  also  in  green- 
houses in  summer  were  it  not  for  the  shading,  which  is  added  to 
reflect  both  the  heat  and  the  light. 

The  reason  why  heat  has  this  effect  upon  growth  is  fairly  well 
known.  Growth  depends  upon  a  number  of  chemical  and  physical 
processes  which  are  kept  in  orderly  cooperation  by  the  protoplasm. 
All  of  these  processes,  in  general,  are  promoted  by  higher  tem- 
perature, which  fact  explains  the  more  rapid  growth  up  to  the 
optimum  point;  but,  as  the  temperature  rises  higher,  to  degrees 
beyond  those  to  which  the  plant  is  accustomed,  the  processes  get 
beyond  control  of  the  protoplasm,  or  run  away,  so  to  speak,  thus 
injuring  and  finally  destroying  the  coordination  and  stopping  the 


334  The  Living  Plant 

growth.  Other  things,  also,  contribute  to  the  result  without 
doubt,  such  as  the  commencement  of  injurious  chemical  reactions 
under  the  higher  temperature,  and  the  accumulation  of  the  waste 
products  which  are  formed  faster  than  they  can  be  removed. 
But  in  general  the  relations  existing  between  temperature  and 
growth  are  determined  by  the  power  of  the  plant  to  control  the 
chemical  and  physical  processes  concerned. 

Second,  as  to  the  effects  of  light  upon  growth.  At  first  thought 
one  would  suppose  that  plants  must  grow  best  in  bright  light, 
since  light  is  essential  to  the  making  of  their  food,  which  supplies 
both  the  material  and  the  energy  for  their  growth;  but  in  truth 
it  is  usually  more  rapid  in  darkness.  This  fact  is  brought  out  in 
our  graph  (figure  123),  though  here,  as  is  usually  the  case,  the 
matter  is  much  complicated  because  the  temperature  commonly 
falls  so  greatly  at  night  as  to  neutralize  any  tendency  the  plant 
may  possess  to  grow  faster  at  that  tune.  But  when  the  {empera- 
ture  remains  even,  as  happens  at  times  on  warm  nights  out  of 
doors,  and  in  greenhouses  artificially  heated,  then  most  plants 
show  a  tendency  to  grow  faster  in  darkness.  These  are  the  con- 
ditions under  which  the  farmer  comments  on  the  great  growth 
that  his  cucumbers,  for  example,  made  in  the  preceding  night. 
Plants  make  ample  food  in  the  day  to  supply  the  growth  through 
the  night.  When,  however,  plants  are  kept  continually  in  the 
darkness  for  days  together,  their  growth  becomes  spindling  and 
weak,  and  their  chlorophyll  disappears,  as  our  picture  will  illus- 
trate (figure  125).  The  results  of  such  growth  are  comparable, 
in  general,  with  the  weakening  activity  of  a  fever. 

The  reasons  why  plants  grow  best  in  the  dark  are  several.  A 
part  of  this  growth  consists  in  that  adaptive  lengthening  (the 
"drawing"  of  gardeners)  already  considered  in  our  third  chapter, 
whereby  plants  reach  up  after  light.  It  is  well  illustrated  by  the 
great  length  of  the  stems  in  our  picture  (figure  125).  A  part  may 
result  from  the  fact  that  during  the  day  all  other  processes  are 
subordinated  to  photosynthesis,  while  at  night  growth  has  the 


Ways  in  Which  Plants  Increase  in  Size 


335 


field  to  itself.  A  part  depends  on  direct  injury  done  by  bright 
light  through  the  injurious  chemical  reactions  set  up  in  the  com- 
plicated protoplasm, — a  matter  we  have  considered  pretty  fully 
under  Protection.  On  green  plants,  of  course,  the  action  of  light 
is  far  less  injurious  than  on  colorless  kinds,  because  the  chloro- 
phyll incidentally  forms  an  excellent  protective  screen.  In  chief 
part,  however,  the  lesser  growth  of  plants  in  light  is  due  to  the 


FIG.  125. — Pots  of  Scilla,  started  alike;  but  that  on  the  right  was  kept  in  a  dark  room. 


great  promotion  of  transpiration  by  the  light  and  its  associated 
heat,  whereby  so  much  water  is  removed  from  the  plant  as  to  lessen 
the  supply  available  for  swelling  the  growing  cells, — for  such  swell- 
ing is  essential  to  their  growth,  as  will  be  noted  more  fully  a  few 
pages  later. 

Thus,  it  is  plain  that  light,  like  heat,  can  become  too  strong  for 
the  best  growth  of  plants.  We  have  seen  already  that  even  in 
photosynthesis  plants  cannot  make  use  of  all  the  light  supplied  by 
direct  bright  sunlight.  These  facts  together  explain  why  so  many 


336  The  Living  Plant 

plants  thrive  better  in  some  shade  than  in  full  sun;  and  it  is  inter- 
esting to  note  that  man  finds  it  best  to  temper  the  light  for  some 
of  his  crops.  This  is  the  reason  why  shading  is  placed  upon  green- 
houses in  summer,  and  why  better  tobacco  is  grown  under  light 
cotton  tents  than  in  full  sun,  though  here  the  protection  given  by 
the  tents  against  hail  storms  and  wind  is  also  important.  In 
Florida,  pineapples  grow  better  under  a  lattice  work  shade  than 
in  full  open  sun. 

Third,  as  to  the  effects  of  humidity  upon  growth.  A  full  supply 
of  water  in  the  soil  is  essential  to  the  process,  for  this  is  the  source 
of  the  water  used  in  swelling  the  small  new  cells  to  their  full  adult 
size.  But  in  addition  the  amount  of  moisture  in  the  air  has  an 
important  influence.  Most  people  know  that  plants  grow  best 
on  the  kind  of  day  we  call  "muggy,"  i.  e.,  one  in  which  the  air 
is  humid,  even  to  the  point  of  discomfort  for  us;  and  it  is  a  familiar 
experience  that  upon  such  a  day  the  grass  of  a  lawn  fairly  grows 
before  the  eyes.  The  influence  of  humidity  in  promoting  growth 
can  also  be  traced  in  our  graph  (figure  123),  which  shows  that  in 
general  growth  increases  with  atmospheric  humidity.  The  chief 
reason  for  this  relation  is  easily  found.  Increased  humidity 
checks  transpiration,  and  therefore  leaves  in  the  plant  a  larger 
water  supply  for  use  in  swelling  the  growing  cells. 

Fourth,  as  to  other  influences  which  affect  growth.  These  are 
few  and  comparatively  unimportant.  Electricity,  applied  ex- 
perimentally in  limited  amount,  stimulates  growth  to  a  certain 
extent  but  in  larger  amount  checks  it;  but  its  influence  is  not 
wholly  separable  from  that  of  heat,  and  the  matter  is  not  so  very 
important,  since  plants  are  hardly  at  all  exposed  to  it  in  Nature. 
Poisonous  substances  in  soil  or  atmosphere  often  stimulate  growth 
a  little  at  first,  though  ultimately  they  check  it,  through  the  in- 
jury they  do  to  the  living  protoplasm.  The  presence  of  a  little 
ether  in  the  air  seems,  however,  to  promote  growth  without  sub- 
sequent detriment,  though  the  reason  for  this  effect  is  not  under- 
stood. The  varying  pressure  of  the  atmosphere,  recorded  by  the 


Ways  in  Which  Plants  Increase  in  Size  337 

barometer,  should  theoretically  have  some  slight  effect,  but 
hardly  any  is  appreciable  in  practice. 

If  now,  we  return  to  the  graph  of  growth  (figure  123)  and  pro- 
ceed to  eliminate  those  fluctuations  which  are  traceable  to  tem- 
perature, light,  and  moisture,  there  still  remains  one  peculiarity 
of  much  consequence,  viz.,  a  gradual  rise  in  the  graph  as  a  whole, 
followed  by  a  more  abrupt  descent.  This  means  that  the  flower- 
stalk  of  the  Grape  Hyacinth,  even  when  all  disturbing  external 
factors  are  eliminated,  does  not  by  any  means  grow  at  a  uniform 
rate  from  start  to  finish,  as  one  might  naturally  suppose,  but,  after 
beginning,  grows  faster  and  faster  up  to  a  point  of  highest  rate, 
beyond  which  its  growth  is  slower  and  slower  until  it  stops.  This 
peculiarity  of  growth,  however,  is  not  confined  to  the  flower-stalk 
of  this  plant,  but  is  very  wide  spread;  and  it  has  even  a  name  of 
its  own,  viz.,  the  "  grand  period."  Thus,  it  is  characteristic  of 
winter  buds;  and  this  explains  a  phenomenon  in  connection  with 
their  opening  which  most  people  must  have  noticed,  viz.,  that 
buds  swell  very  slowly  at  first  in  the  spring,  seeming  to  take  an 
interminable  tune  before  they  show  their  green  leaves,  after 
which  they  open  out  very  quickly,  almost  over  night  as  it  seems, 
to  nearly  the  full  size  of  their  parts;  and  then  they  complete  their 
final  growth  rather  slowly.  This  opening  takes  place  on  the  crest 
of  the  grand  period  as  a  rule,  although  it  is  complicated  of  course 
more  or  less  by  the  effects  of  temperature.  Leaves,  single  flowers, 
germinating  embryos,  fruits,  and  a  good  many  other  parts  display 
the  grand  period.  It  is  not,  however,  universal;  for  some  struc- 
tures, like  stems  which  continue  their  growth  all  summer,  pursue 
an  even  course  affected  only  by  varying  temperature,  moisture, 
or  light. 

By  suitable  modifications  in  details,  records  may  also  be  se- 
cured by  the  auxograph  for  the  growth  of  leaves  and  of  roots. 
The  graphs  in  general  are  very  similar  to  those  obtained  from 
stems.  But  there  is  one  feature  of  the  growth  of  leaves,  stems  and 
roots,  about  which  the  auxograph  gives  no  information, — namely, 


338 


The  Living  Plant 


the  place  of  most  active  growth  in  each  part,  whether  at  tip,  base, 
or  all  through  the  structure.  This,  however,  is  easily  determined 
in  another  way,  viz.,  by  marking  the  parts  when  young  by  evenly- 
spaced  lines,  the  spread  of  which,  as  the  parts  grow  up,  must 

reveal  the  place  where  these 
grow  the  most.  If  a  young 
root  be  thus  marked  by  cross 
lines,  the  result  is  like  that  of 
our  figure  (figure  126).  Evi- 
dently young  roots  grow  almost 
wholly  at  their  tips.  If  stems 
be  marked  in  the  same  way,  the 
result  is  somewhat  different 
(figure  127).  Evidently  young 
stems  grow  mostly  at  their 
tips,  but  over  a  much  larger 
area  than  the  roots,  as  indeed 
one  might  infer  from  the  way  in 
which  the  nodes  of  young  stems 
spread  apart.  It  is  no  trouble  at  all  to  find  an  adaptive  reason  for 
the  difference  in  the  mode  of  growth  of  roots  and  stems,  when  one 
recalls  that  roots  must  pick  their  way  through  the  irregular  in- 
terstices of  a  closely-pressing  soil,  while  stems  have  all  outdoors 
to  expand  in.  As  to  leaves,  their  shape  makes  it  necessary  to 
mark  them  by  cross  lines,  forming  squares,  and  when  thus  treated 
the  spread  of  the  lines  shows  that  leaves,  unlike  roots  and  stems, 
grow  all  through  their  structure  (figure  128).  Slender  leaves, 
however,  especially  the  kind  that  grow  up  from  bulbs,  grow  al- 
most wholly  at  the  base. 

Although  growth  is  typically  accompanied  by  increase  in 
length,  it  sometimes  is  correlated  with  shortening.  One  case 
thereof  is  found  where  a  straight  structure  becomes  a  spiral, 
as  in  tendrils,  which  thus  pull  their  plants  closer  to  a  support, 
or  in  the  peduncles  of  some  water  plants,  which  thus  draw  their 


FIG.   126. — A  young    Bean    root,      \ 
just  marked   by   evenly  spaced 
cross  marks,  and  the  same  root 
a  day  later. 


Ways  in  Which  Plants  Increase  in  Size 


339 


ripening  fruits  to  a  safer  position  under  water.  But  an  actual 
shortening  occurs  in  the  roots  of  some  herbaceous  perennials, 
like  the  Dandelion,  which  thus  are  enabled  to  keep  their  stems 
safely  underground  despite  a  certain  annual  increase  in  length. 


FIG.  127.— A  stem 
of  Melothria,  just 
marked  by 
evenly-  spaced 
cross  marks,  and 
the  same  stem  a 
day  or  two  later. 


The  same  thing  is  said  to  occur  in  the  lateral  rootlets  of  some 
bulb-bearing  plants,  like  the  Tulips,  with  this  marked  advantage, 
that  the  newly  formed  bulblets  are  drawn  clear  of  the  old  parent 
bulb.  Mechanically,  this  shortening  is  variously  effected,  but 
chiefly  by  a  forcible  lateral  expansion  of  the  tissues,  somewhat  on 
the  principle  by  which  a  muscle  is  shortened;  and  as  a  result 
such  roots  commonly  show  a  number  of  transverse  wrinkles. 


340  The  Living  Plant 

Such  are  the  principal  phenomena  of  that  phase  of  growth 
which  is  concerned  with  enlargement.  Another  phase  is  con- 
cerned with  the  formation  of  new  parts,  or  development.  But 
the  relations  of  the  two  will  be  much  plainer  if,  before  proceeding 
with  the  latter,  I  describe  the  cellular  basis  of  both.  As  to  this, 
we  may  anticipate  a  little  by  saying  that  in  general,  enlargement 


FIG.  128. — A  young  leaf  of  English  Ivy  marked  in  regular  squares,  and  the  same  leaf  a 
week  or  two  later. 

depends  upon  swelling  of  cells  already  formed,  while  develop- 
ment, or  the  construction  of  new  parts,  rests  upon  the  formation 
of  new  cells. 

The  mode  of  formation  of  new  cells  is  singularly  uniform 
throughout  all  plants.  It  takes  place,  as  a  rule,  only  in  small 
compact  thin-walled  cells  densely  filled  with  protoplasm,  the 
kind  technically  known  as  meristem  and  best  shown  at  the  growing 
points  of  stems  and  roots  (figures  53,  137,  139,  C.  D).  The  details 
cannot  be  seen  in  living  cells,  but  can  be  inferred  from  the  appear- 
ances presented  by  cells  killed,  stained,  and  sectioned  for  the  pur- 
pose. The  first  sign  of  new  cell  formation  occurs  in  the  nucleus 
(figure  101),  where  the  granules  become  more  conspicuous  and  col- 
lect into  stout  threads  which  then  sort  themselves  out  in  the  form 


Ways  in  Which  Plants  Increase  in  Size  341 

of  a  definite  number  of  the  bodies  called  chromosomes;  and  these 
become  arranged  in  a  plate  across  the  cell.  Meanwhile  the  bound- 
ary of  the  nucleus  has  vanished,  and  a  spindle-shaped  framework 
of  very  fine  fibers  has  formed  at  right  angles  to  the  chromosome 
mass.  Then  each  chromosome  splits  lengthwise  into  two,  and  the 
spindle  draws  these  halves  apart  towards  its  two  ends,  where 
they  become  surrounded  anew  by  a  nuclear  boundary.  Thus  is 
the  chromosome  matter  divided  evenly  between  the  two  new 
nuclei.  The  chromosomes  then  lose  their  distinctness  and  grad- 
ually merge  away  to  the  threads,  and  finally  to  a  granulation 
similar  to  that  of  the  original  nucleus.  Meantime  the  spindle 
fades  away  and  a  new  wall  forms  across  the  cell  between  the  new 
nuclei.  Each  of  the  new  cells  then  grows  to  the  original  size  and 
is  ready  for  another  division. 

The  object  of  this  elaborate  process  is  without  doubt  the  equal 
division  of  the  chromosomes.  These,  it  will  be  remembered,  are 
derived  equally  from  the  two  parents  of  the  plant,  half  of  them 
from  one  parent  and  half  from  the  other;  and  although  they  ab- 
sorb nourishment  and  grow  and  divide,  they  never  lose  their 
identity.  The  equal  division  of  the  chromosomes  in  every  division 
of  the  cells,  therefore,  carries  some  of  the  substance  derived  from 
each  parent  to  every  cell  of  the  adult  plant,  thus  explaining  how 
it  is  that  any  part  of  a  plant  can  resemble  either  one  of  its  parents. 

Cell  division  underlies  all  development  of  new  parts,  for  every 
structure — leaf,  stem,  root,  or  other — begins  with  the  formation 
of  just  so  many  cells  at  just  such  places  as  will  produce,  when  they 
swell  to  full  size,  the  characteristic  size  and  shape  of  the  fully-adult 
organ.  But  at  first  these  cells  are  all  small,  and  densely  packed 
with  protoplasm  and  food  substance.  Such  is  the  condition  in  a 
bud  or  an  embryo,  as  our  figures  illustrate  (figures  137,  139,  C). 
One  must  not,  however,  lay  too  much  stress  upon  the  cell  divisions 
in  particular,  for  they  are  without  doubt  a  result,  rather  than  a 
cause,  of  the  outgrowth  of  new  parts.  It  is  in  reality  the  living 
protoplasm  which  pushes  out  into  new  structures;  the  cell  divi- 


342  The  Living  Plant 

sions  take  place  as  a  secondary  architectural  arrangement.  It  is 
easy  to  follow  the  method  whereby  the  individual  cells  grow  from 
the  tiny  food-packed  condition  to  the  large  protoplasm-lined  and 
water-filled  state  that  distinguishes  them  when  adult;  and  the 
matter  is  well  illustrated  in  the  accompanying  figure  129.  First 
of  all,  inside  the  dense  protoplasm  there  appear  little  rifts  which 
contain  a  sugar-rich  sap.  Into  these  little  sap-cavities  water  is 
absorbed  osmotically,  making  them  swell  and  exert  pressure 
which  pushes  the  protoplasm  against  the  walls  and  stretches  them 
tensely.  But  this  pressure  is  relieved  by  the  deposition  of  new 
substance  all  through  the  innermost  texture  of  the  stretched  wall ; 
and  this  allows  a  still  further  stretching,  and  so  on  until  the  cell  is 


FIG.  129. — Generalized  drawings,  in  optical  section,  of  a  cell  during  enlargement  from  the 
newly  developed  to  the  fully-adult  condition. 

full  grown.  The  sap-cavities,  meanwhile,  are  not  only  enlarging 
but  are  merging  together;  and  the  food  substance  originally 
stored  in  the  cell  is  being  transformed  into  new  cell-wall,  proto- 
plasm, and  materials  dissolved  in  the  sap.  The  final  product  is  a 
fully-grown  cell,  many  times  larger  than  its  embryonic  original 
and  provided  with  a  tightly-stretched  wall  against  which  lies 
a  thin  lining  of  protoplasm,  enclosing  a  single  sap-cavity  well- 
nigh  as  big  as  the  cell  itself.  The  exact  direction  of  expansion  of 
the  cell,  and  therefore  its  final  shape,  are  of  course  by  no  means 
accidental,  but  are  under  control  of  the  living  protoplasm,  which 
thus  simply  makes  use  of  osmotic  pressure  as  the  mechanical 
power  for  forcing  cell  enlargement.  And  the  degree  to  which 
that  enlargement  may  proceed,  from  the  newly-developed  to  the 


Ways  in  Which  Plants  Increase  in  Size 


343 


fully-adult  cell,  is  sometimes  surprisingly  great,  as  the  accom- 
panying example  well  illustrates  (figure  130). 

From  these  considerations  it  will  be  plain  that  the  fully-adult 
cell  consists  largely  of  water,   with  comparatively  little  solid 
matter,  in  great  contrast  to  the  embryonic  cell  which  is  largely 
solid.    This  is  shown  very  clearly  by  the  great 
collapse  of  fresh  plant-structures  when  dried  \\ 

(for  often  they  shrink  away  to  a  mere  wisp  of 
their  former  selves),  and  also  by  weighings, 
which  prove  that  most  fresh  plant-structures 
consist  of  more  than  90  per  cent  water.  A 
plant  as  large  as  that  shown  in  our  figure,  for 
example,  (figure  131),  can  be  contained  when 
dried  in  the  tiny  vial  beside  it.  The  same  thing 
is  true  also  of  seedlings  and  the  spring  vegeta- 
tion from  buds;  when  the  water  is  expelled,  it 
is  found  that  the  fully  grown  structure  is  not 
only  no  heavier  than  the  embryo  or  bud,  but 
even  lighter  in  weight, — the  loss  of  course  be- 
ing due  to  the  removal  of  material  by  respira-  FIG.  iso.— The 
tion.  Thus  in  general  it  is  true  that  developing 
structures  gain  weight,  while  growing  struc- 
tures lose  it. 

That  growth  consists  chiefly  in  swelling  of 
cells  already  laid  down  in  development  is 
shown  very  beautifully  by  comparison  of  some 
embryos  with  the  seedlings  that  grow  from  them.  If  cross- 
sections  of  embryos  and  seedlings  be  made  in  about  the  same 
place,  it  is  found  on  the  average  that  although  the  cells  differ  very 
greatly  in  size,  their  number  is  approximately  the  same,  though 
in  one  case  they  are  tiny,  squarish,  densely  packed  and  full  of 
substance,  while  in  the  other  they  are  large,  rounded,  loosely- 
arranged,  and  contain  little  but  water.  This  separation  of  devel- 
opment and  growth  is  more  common  than  one  would  suppose,  for 


parative  sizes  of  a 
pith  cell  in  the  newly 
developed  and  the 
fully-adult  condition, 
as  seen  in  optical 
section.  Traced  from 
accurate  drawings  on 
a  wall-chart  by  Frank 
and  Tschirch. 


344 


The  Living  Plant 


even  in  structures  which  grow  on  continuously,  and  in  which  it 
would  seem  that  the  two  phases  must  be  mixed  up  together,  they 
are  separated  in  space,  although  not  in  time.  Thus,  in  roots,  the 
development  of  new  cells  occurs  in  the  growing  point  (figure  53, 
139,  D),  while  the  enlargement  of  cells  to  full  size  takes  place  in  the 


FIG.  131. — A  Castor  Bean  plant,  with  its  dry  substance  in  a  vial  alongside.     (The  vial,  of 
course,  was  photographed  later,  and  worked  into  the  plate.) 


zone  just  behind,  a  fact  which  explains  the  enlargement  of  that 
zone  as  shown  in  our  earlier  figures.  The  same  is  true  likewise  of 
the  stem,  though  less  strikingly.  Moreover,  it  is  also  a  very  inter- 
esting fact  that  if  a  plant  is  suddenly  called  upon  to  increase  be- 
yond the  normal,  as  for  example  in  the  longer  leaf-stalks  demanded 
of  water  plants  forced  to  grow  in  deeper  water,  or  in  the  leaves  of 
plants  whose  buds  have  all  been  destroyed,  the  enlargement  is 


Ways  in  Which  Plants  Increase  in  Size  345 

attained  by  increasing  the  size  of  cells  beyond  the  normal,  not  by 
increasing  their  number. 

But  while  enlargement  and  development  are  separate  in  their 
nature,  and  commonly  occur  apart  from  one  another,  neverthe- 
less they  are  often  intermingled  more  or  less.  The  very  act  of 
development,  indeed,  entails  some  increase  of  size,  and  enlarge- 
ment is  attended  by  some  cell  divisions  in  connection  with  adjust- 
ment of  parts;  and  no  doubt,  furthermore,  there  are  structures 
which  develop  and  enlarge  simultaneously. 

As  growth  comes  near  to  completion,  and  sometimes  much 
earlier,  the  cells  undergo  such  further  changes  as  fit  them  more 
perfectly  for  particular  functions.  Such  changes  are  designated 
maturation.  Walls  thicken  in  places  and  are  absorbed  in  others; 
they  develop  spirals,  rings,  or  other  thickenings,  and  hollow  pits 
or  other  depressions;  while  various  changes  take  place  as  well  in 
the  contents,  which  often  are  transformed  to  secretions  of  very 
specialized  function.  Moreover,  as  cells  increase  in  perfection  of 
adaptation  to  their  functions,  they  lose  at  the  same  time  their 
power  of  division,  so  that  when  fully  mature  they  are  incapable 
of  further  development  or  reproduction.  But  these  changes  in 
the  main  have  already  been  considered  in  the  chapters  on  Pro- 
toplasm and  Metabolism. 

Before  leaving  this  aspect  of  growth,  we  should  summarize  for 
completeness  the  other  physical  and  chemical  phenomena  thereof, 
most  of  which  have  been  considered  in  various  connections  earlier 
in  this  book.  Thus,  there  is  always  a  large  conversion  of  stored 
food  into  new  walls,  protoplasm,  and  sap  substances,  resulting  in 
the  collapse  of  the  storage  parts  of  sprouting  structures,  like 
potatoes,  bulbs  and  seeds.  Again,  respiration,  the  releaser  of 
energy,  is  indispensable  to  growth,  which  demands  much  of  it; 
and  so  close  is  the  connection  of  the  two,  that  whatsoever  stops 
the  one  stops  also  the  other.  Therefore,  oxygen  being  essential  to 
respiration,  if  the  oxygen  supply  be  cut  off  from  the  growing 
plant,  as  happens  often  in  nature  through  flooding  with  water, 


346 


The  Living  Plant 


and  as  can  easily  be  effected  by  experiment,  then  growth  ceases; 

and  indeed  death  ensues  unless  the  supply  be  admitted  again. 

Furthermore,  in  the  chemical  reactions  of  growth,  some  waste 

by-products  are  formed,  of  which  a  part  are  dropped  with  the 

bark  and  the  leaves,  a  part 
are  stored  in  out  of  the  way 
cells,  and  a  part  are  appar- 
ently excreted  into  the  soil, 
where  they  act  poisonously, 
and  produce  economic  and 
ecological  consequences  al- 
ready described. 

A  notable  feature  of  growth 
is  its  accompaniment  by  a 
number  of  different  move- 
ments. Many  of  these  are 
clearly  adjustive  of  the  parts 
to  the  particular  conditions 
of  light,  moisture,  and  so  forth 
prevailing  in  the  immediate 

Fio.    132.— An   arrangement   (about  one-sixth  environment,      and      as      SUCH 
the  true  size),  for  the  study  of  circumnuta-  -,  •  j  i      i         j 

tion,  using  a  method  described  in  the  text,  have  been  considered  already 

The  glass  filament  and  paper  triangle,  some-  m  our  chapter  On  Irritability, 
what  exaggerated  for  visibility  in  the  draw-  J  ' 

ing,    may   be  seen   near  the  center  of   the  while    they    will    also    receive 
picture.  . 

further   mention  in    suitable 

places  in  the  chapter  that  follows.  There  is,  however,  one  move- 
ment of  which  the  description  belongs  here,  since  it  is  an  inciden- 
tal accompaniment  of  all  growth.  It  is  that  which  was  named  by 
Darwin,  its  discoverer,  circumnutation.  So  slow  is  it,  ordinarily, 
however,  that  special  methods  are  needed  to  render  it  apparent. 
If  one  takes  some  young  seedling,  such  as  Radish  or  Corn,  attaches 
alongside  its  tip  by  harmless  cement  a  slender  projecting  glass  fila- 
ment, places  black  reference  marks  on  the  end  of  the  filament  and 
on  a  bit  of  white  paper  at  its  base,  and  then  supports  a  pane  of 


Ways  in  Which  Plants  Increase  in  Size  347 

glass  horizontally  a  foot  above  it  (figure  132),  he  can,  by  sighting 
his  reference  marks,  record  on  the  pane  the  spot  to  which  the  fila- 
ment is  then  pointing.  But  if,  a  half  hour  later,  he  sights  again, 
he  finds  that  the  filament,  and  therefore  the  tip  of  the  plant, 
points  in  another  direction,  and  later  in  another,  and  so  on.  By 
drawing  straight  lines  through  the  points  thus  established,  one 
obtains  a  kind  of  polygon  representative  crudely  of  the  magnified 
course  of  the  moving  tip  of  the  seedling;  and  a  few  of  these  records, 
traced  by  one  of  my  own  students,  are  given  herewith  (figure  133), 
while  Darwin's  book,  The  Power  of  Movement  in  Plants,  contains 
a  great  number.  These  are  not  by  any  means  isolated  cases,  for 
comparative  studies  have  shown  that  such  movements  are  dis- 
tinctive of  most  if  not  all  growing  parts, — stems,  buds,  leaves, 
roots,  tendrils,  flowers  and  their  parts,  and  many  others, — all  of 
which  move  during  growth  in  slow,  irregular,  and  jerky  paths, 
that  are  longer  and  more  rapid  the  more  active  the  growth  of  the 
part.  While  the  movement  is  thus  well-nigh  universal,  it  is  not 
popularly  known  because  of  its  slowness.  If  its  rate  could  be 
magnified  a  few  dozens  of  times,  what  a  different  aspect  would 
vegetation  present!  Then  all  the  visible  parts  of  all  the  growing 
plants  of  a  garden,  a  meadow,  or  a  forest,  would  exhibit  a  con- 
stant irregular  movement,  which  collectively  would  seem  of  a 
tremulous  character, — much,  I  imagine,  as  would  be  shown  if 
the  plants  were  shaken  by  continuous  little  earthquakes. 

As  to  the  cause  of  the  circumnutation,  that  is  known,  in  prin- 
ciple at  least.  It  results  from  the  fact  that  all  growing  structures, 
utilizing  as  they  do  osmotic  turgescence  for  the  expansion  of  their 
tissues,  are  under  strong  internal  pressures  which  hold  them  hi  a 
highly  tense  but  unstable  stiffness.  Now  the  readjustment  of 
these  pressures  in  growth  cannot  proceed  with  perfect  evenness  all 
around  the  stems  or  other  parts,  whose  great  length  and  slender- 
ness  cause  a  large  magnification  of  even  the  slightest  disturb- 
ances of  the  equilibrating  tensions, — and  circumnutation  results. 
These  movements,  therefore,  are  simply  an  incidental  by-product 


348 


The  Living  Plant 


FIG.  133. — Records  of  the  circumnutation  of  some  common  plants,  obtained  by  the  method 
illustrated  in  figure  132.  The  letters  h  and  m  signify  hours  and  minutes  between 
observations. 

of  growth,  and  one  of  those  incidental  phenomena  which  possess 
no  adaptational  significance;  and  it  is  partly  because  it  is  so  good 
an  example  of  such  incidental  phenomena  (of  which  autumn 
coloration,  forms  of  starch  grains,  and  phyllotaxy,  are  instances 


Ways  in  Which  Plants  Increase  in  Size  349 

earlier  described  in  this  book),  that  I  give  to  it  here  so  much  atten- 
tion. There  is,  however,  another  reason  for  its  consideration, 
namely,  that  Darwin  considered  it  the  starting  point  for  most  of 
the  useful  plant  movements, — the  twining,  sleep,  geotropic,  hydro- 
tropic  and  other  adjustive  movements  which  we  considered  under 
Irritability.  His  conclusion  on  this  point,  has  not,  however,  been 
accepted  by  later  investigators,  though  the  present  status  of  the 
matter  may  be  expressed  by  saying  that  his  view  is  unproven 
rather  than  disproven. 

Finally,  as  to  the  physiological  phases  of  growth,  there  remains 
one  matter  which  is  both  scientifically  interesting  and  econom- 
ically important, — and  it  concerns  grafting.  Everybody  knows 
that  small  twigs  of  apples,  cherries,  pears  and  many  other  plants 
can  be  cut  from  those  trees  and  inserted  into  the  stems  of  others 
in  such  way  as  to  grow  and  form  structurally  an  integral  part  of 
the  new  tree.  Furthermore  (and  this  is  what  gives  to  grafting  its 
great  economic  importance),  the  inserted  twig  and  everything 
which  subsequently  grows  from  it,  continues  to  produce  its  own 
kind  of  leaves,  flowers,  and  fruits  substantially  unaffected  by  the 
plant  into  which  it  was  grafted;  while,  correlatively,  the  stock 
plant  into  which  the  graft  was  inserted  continues  to  produce  its 
own  kind  of  vegetation  unaffected  by  the  graft,  even  though  this 
may  in  time  become  the  greater  part  of  the  tree.  Thus  it  is 
possible  to  graft  a  number  of  very  different  varieties  of  apples,  or 
of  cherries,  into  a  single  trunk  and  produce  a  tree  which  bears 
all  those  varieties  as  long  as  it  lives,  without  any  visible  sign  to 
show  that  it  was  ever  anything  other  than  one  tree  from  the  start. 
It  is  in  this  way  that  highly  specialized  forms  of  fruits,  leaves,  or 
flowers,  which  appear  mainly  as  sports  (to  be  further  considered 
in  our  chapter  on  Plant  Breeding),  and  which  cannot  be  grown 
from  seed,  are  propagated  and  multiplied  indefinitely. 

Turning  now  to  the  purely  physiological  side  of  grafting,  the 
first  fact  of  prominence  is  this,  that  the  twig,  which  is  called  the 
scion,  (or  don),  and  the  plant  into  which  it  is  inserted,  called  the 


35Q  The  Living  Plant 

stock,  must  be  closely  related,  else  no  union  of  tissues  takes  place. 
We  find  the  same  necessity  in  hybridization,  or  crossing  of  dif- 
ferent varieties  of  species  by  pollination;  and  indeed  the  possibili- 
ties of  grafting  and  of  hybridization  have  much  the  same  limits, 
being  comparatively  easy  between  varieties  of  one  species,  much 
less  so  between  species  of  the  same  genus,  extremely  rare  between 
different  genera,  and  unknown  outside  of  the  same  family.  Prob- 
ably the  reason  is  a  chemical  one — the  more  distantly  related  the 
forms  the  more  likely  is  their  protoplasm  to  contain  chemicals 
which  react  on  one  another  in  a  way  to  produce  disturbing  if  not 
injurious  or  fatal  compounds,  thus  preventing  a  normal  or  orderly 
continuance  of  growth.  But  when  the  protoplasm  of  scion  and 
stock  is  actually  congenial,  so  to  speak,  then  the  two  grow  to- 
gether precisely  as  a  wound  on  one  plant  would  heal  up,  and  the 
tissues  unite  and  thereafter  grow  as  one  single  mass.  It  is  neces- 
sary that  a  considerable  area  of  living  tissue  be  brought  into  con- 
tact, which  is  comparatively  easy  in  these  plants  possessing  a 
cambium  cylinder  (i.  e.  a  continuous  growth  system  soon  to  be 
described),  but  it  is  practically  impossible  in  others.  This  fact 
explains  why  no  grafting  is  possible  among  plants  belonging  to  the 
groups  of  the  Corn,  Lilies,  Palms,  wherein  no  cambium  exists. 

Although,  in  general,  the  scion  and  stock  retain  each  its  own 
characters  unaffected  by  the  other,  a  partial  exception  occurs  in 
some  minor  features,  such  as  earliness  of  blossoming,  resistance  to 
frost,  and  even  some  slight  alterations  in  flavor  of  fruit  or  its 
color.  In  all  these  cases,  I  believe,  such  characters  can  be  traced 
to  the  influence  of  the  sap,  which  of  course  moves  from  stock  to 
scion,  or  of  the  food  substance,  which  moves  from  scion  to  stock. 
The  living  protoplasm,  however,  does  not  thus  move  from  one  to 
another,  but  remains  within  the  original  cells,  or  those  which 
grow  from  them;  wherefore  the  characters  which  depend  on  the 
protoplasm,  including  substantially  all  of  those  which  give  the 
distinctive  characteristics  to  plants,  are  never  transferred  from 
stock  to  scion,  or  vice  versa. 


Ways  in  Which  Plants  Increase  in  Size  351 

From  the  facts  just  stated,  it  would  seem  impossible  for  graft 
hybrids,  that  is,  intimate  mixtures  of  the  protoplasm  of  stock  and 
scion,  to  exist.  Yet  graft  hybrids  have  actually  been  claimed  to 
occur,  though  very  rarely.  And  here  opens  up  one  of  the  most 
interesting  chapters  in  recent  experimental  studies,  for  it  has  been 
found  possible  to  produce  experimentally  such  apparent  graft 
hybrids.  But  the  very  same  experiments  have  shown  that  they 
are  really  not  hybrids  at  all,  but  merely  mixtures  of  the  tissues  of 
the  scion  and  stock,  and  not  a  blending  of  their  protoplasm.  These 
experiments  were  made  by  grafting  a  part  of  a  bud  of  the  scion  to 
a  part  of  a  bud  of  the  stock,  when  the  resultant  branch  displayed 
a  most  remarkable  mixture  of  the  colors,  shapes,  tissue  characters, 
and  other  features  of  scion  and  stock — not  a  blending  but  a 
mixture.  Sometimes  the  upper  side  of  the  branch  would  be  all 
scion  with  the  characters  thereto  appropriate,  and  the  under  side 
all  stock;  sometimes  a  sheath  of  stock  enwrapped  a  core  of  scion; 
and  other  mixtures  of  other  sort  occurred.  Such  graft  products 
are  not  hybrids,  and  have  been  named  chimceras.  But  are  graft 
hybrids  then  impossible?  Theoretically  they  are  not,  for  if  one 
cut  cell  of  the  stock  and  one  cut  cell  of  the  scion  should  happen  to 
match  together,  and  if  then  their  two  nuclei  should  fuse  together 
(as  they  well  might,  for  we  know  cases  of  fusion  of  nuclei  other 
than  in  fertilization) ;  and  if  from  this  hybrid  cell  there  should 
then  develop  a  branch  by  the  ordinary  process  of  cell  division, 
then  the  cells  of  that  branch  would  all  possess  protoplasm  and 
chromosomes  from  both  stock  and  scion,  and  a  true  graft  hybrid 
would  exist.  This  alluring  possibility  has  naturally  attracted 
the  eager  attention  of  the  experimenters,  and  already  they  have 
announced  success,  though  as  yet  of  a  somewhat  unsatisfying 
character.  And  if  by  good  fortune  I  have  ever  the  privilege  of 
preparing  a  new  edition  of  this  book,  I  shall  probably  be  able  to 
describe  much  that  is  important  and  interesting  in  this  connec- 
tion; for  this  line  of  experimentation  has  opened  up  much  more 
than  merely  this  question  of  graft  hybrids. 


CHAPTER   XIV 

THE  ORDERLY  CYCLES  PURSUED  IN  GROWTH,  AND 
THE  REMARKABLE  RESULTS  OF  DISTURBANCE 
THEREOF 

Growth:  structural 

HE  reader  may  possibly  wonder,  as  he  contemplates  the 
chapter  before  him,  what  reason  there  is  for  its  separa- 
tion from  the  one  that  precedes  it,  when  both  are  con- 
cerned with  the  very  same  subject  and  closely  inter- 
connected. So  I  may  as  well  make  the  confession  that  it  has  not  a 
much  better  basis  than  the  reason  assigned  by  an  early  French 
naturalist  for  excluding  the  Crocodiles  from  Insects, — the  animal 
seemed  to  belong  there,  but  would  make  quite  too  terrible  an 
insect!  I  like  to  conceive  of  this  book  as  read  one  chapter  at  a 
sitting  by  a  reader  who  has  interest  enough  in  the  subject  to  make 
its  careful  perusal  the  chief  feature  of  an  evening's  business;  and 
so  much  must  be  said  about  growth  that  it  cannot  be  followed 
unweariedly  without  some  kind  of  division  or  intermission. 
However,  the  matter  is  really  not  quite  so  desperate  as  this,  for 
the  physiological  and  structural  phenomena  of  growth  are  in  fact 
sufficiently  different  to  make  a  division  between  them  not  wholly 
unnatural. 

Of  the  structural  phenomena  of  growth,  the  most  striking  and 
important  are  concerned  with  the  cycle  of  development  of  the 
individual  plant  from  its  very  first  origin  up  to  its  adult  condition; 
and  this  is  comprised  in  four  stages. 

1.  The  Growth  Cycle;  from  Egg-cell  to  Embryo. — This  stage 
is  rather  well  represented,  albeit  somewhat  diagrammatically,  by 

352 


The  Orderly  Cycles  Pursued  in  Growth 


353 


the  accompanying  picture  (figure  134).  The  reader  will  recall 
that  the  egg-cell  is  the  female  reproductive  cell  formed  inside  the 
embryo-sac  within  the  ovule,  and  that  it  needs  to  be  fertilized  by 
a  male  cell  brought  by  a  pollen-tube,  before  it  can  develop  to  an 
embryo.  Immediately  after  fertilization,  the  egg-cell  divides  into 
two;  these  grow  in  size,  and  again  divide,  and  so  on  in  a  way  to 


FIG.  134. — Typical  stages  in  the  development  of  an  egg-cell  into  an  embryo  (of  Rape). 
The  original  egg-cell  lay  at  the  bottom  of  the  embryo  sac  of  which  a  part  is  shown  in 
the  figure  on  the  left,  while  the  other  figures  show  the  development  of  the  initial  cell, 
at  the  top  of  the  suspensor,  into  the  embryo.  (Adapted  from  pictures  on  a  wall-chart 
by  L.  Kny.) 

produce  a  line  of  cells  forming  a  structure  called  a  suspensor, 
which  carries  a  terminal,  or  initial,  cell  out  into  the  middle  of  the 
embryo-sac,  where  there  is  ample  space  for  the  development  of 
the  forthcoming  embryo.  Then  the  initial  cell  begins  to  divide, 
first  at  right  angles  to  the  earlier  divisions,  then  again  and  again 
in  other  planes  with  great  regularity,  as  represented  in  our  pic- 
tures, until  finally  a  many-celled  ball  is  produced.  Then  the 
regularity  ceases,  and  cell  division  becomes  more  active  at  two 


354  The  Living  Plant 

definite  places,  resulting  in  outgrowths  which  wax  greater  and 
greater  until  they  become  the  thick  leaves  that  later  are  called 
the  cotyledons.  Meanwhile  the  original  ball  is  growing  more 
actively  at  the  opposite  end,  there  producing  a  cylindrical  struc- 
ture which  forms  the  stem,  or 
hypocotyl,  of  the  embryo,  while 
a  group  of  growth  cells  at  its 
tip  forms  the  foundation  for 
the  forth-coming  root,  and 
,  another  between  the  cotyle- 

I-IG.    13o. — Typical   seeds,    with   embryos,   of 

the  two  leading  types;— "albuminous"   (a  dons  forms  the  foundation  for 

Barberry)  and  "  exalbuminous "  (an  Apple) ,  ,        ~                       .       i    i      j        mi 

as  further  explained  in  the  text.      (Copied  the  .IlTSt    terminal    DUQ.       1  hUS 

are  the  first  leaves  and  stem, 

and  the  foundations  for  the  first  root  and  bud,  of  the  new 
plant  laid  down  wholly  inside  the  embryo-sac  of  the  ovule, 
forming  the  structure  which  we  call  the  embryo.  Simultaneously 
the  coats  of  the  ovule  are  growing  thicker  and  harder,  the  suspen- 
sor  is  being  absorbed,  and  a  supply  of  food  substance,  the  endo- 
sperm, developed  in  a  manner  already  described  (page  299),  is 
filling  all  the  space  in  the  embryo-sac  not  preoccupied  by  the 
embryo.  The  resultant  structure,  a  combination  of  embryo,  food 
substance,  and  protective  coats,  is  the  Seed  (figure  135,  on  the 
left). 

Such  is  the  typical  method  of  development  of  embryos  and 
seeds,  though  of  course  a  great  many  differences  occur  in  detail. 
In  some  seeds  the  development  stops  at  the  point  here  described, 
leaving  the  young  embryo  surrounded  by  copious  endosperm  or 
"albumen";  but  in  others,  for  example,  Peas  and  Beans,  the 
embryo  continues  its  development  until  it  has  absorbed  all  the  en- 
dosperm and  everything  else  inside  of  the  seed  coats,  in  which  case 
it  usually  develops  also  the  first  bud,  called  the  plumule,  between 
the  cotyledons  (figure  135,  right).  In  any  case,  the  seed  is  now 
ripe.  It  gives  up  most  of  its  water,  hardens  its  coats,  separates 
from  the  parent  plant,  and  goes  into  that  resting  state,  in  which 


The  Orderly  Cycles  Pursued  in  Growth  355 

some  kinds  may  remain,  with  vitality  intact,  for  years  and  dec- 
ades, and  even  a  century,  though  not  for  the  ages  implied  in  the 
current  but  groundless  belief  that  genuine  seeds  from  the  wrap- 
pings of  mummies  will  germinate.  In  this  condition,  small  and 
light,  and  independent  of  external  food  or  water  supply,  the  seed 
is  capable  of  wide  transport;  and  thus  forms  a  natural  stage 
for  dissemination,  in  adaptation  to  which  its  coats  or  neighboring 
structures  often  develop  wings,  plumes,  hooks,  pulp  and  colors,  as 
we  shall  consider  more  fully  in  the  following  chapter.  It  is  not  of 
course  because  the  seed  has  these  characters  that  it  is  utilized  by 
the  plant  as  its  dissemination  stage,  but  it  is  rather  because  it  has 
been  developed  as  the  dissemination  stage  that  it  has  these  char- 
acters. 

In  following  the  sequence  of  cell  divisions  involved  in  these  re- 
sults one  cannot  but  wonder  what  the  nature  of  the  controlling 
power  must  be.  Structurally  considered,  cell  division  can  take 
place  just  as  well  in  one  direction  as  another,  yet  in  fact  it  takes 
place  in  substantially  the  same  directions  as  in  preceding  genera- 
tions of  embryos, — directions  which  bring  an  adaptive  result. 
What  is  it  which  compels  the  developing  egg-cell  to  form  a  line  of 
cells  instead  of  a  ball,  and  the  initial  cell  to  form  a  ball  instead  of 
a  line;  which  leads  the  ball  to  push  out  the  two  cotyledons  in 
definite  places,  and  to  make  the  hypocotyl  and  root  in  another? 
In  some  way,  it  is  certain,  the  control  issues  from  the  chromo- 
somes, which  alone  hold  the  knowledge  of  how  the  former  genera- 
tions developed;  but  through  what  mechanism  do  they  exert 
their  authority?  This  question,  for  the  most  part,  we  cannot  yet 
answer,  but  in  some  part  we  can;  for  it  seems  reasonably  certain 
that  most  of  the  changes  consist  in  responses  to  stimuli,  the  nature 
of  which  was  explained  in  our  chapter  on  Irritability.  Perhaps 
the  pressure  of  the  egg-cell  against  the  end  of  the  embryo-sac  is 
the  stimulus  which  sends  the  suspensor  developing  as  a  single  cell- 
line  in  the  opposite  direction;  perhaps  the  freer  osmotic  absorption 
permitted  by  the  arrival  of  the  initial  cell  into  the  more  fluid 


356  The  Living  Plant 

central  part  of  the  embryo-sac  is  the  stimulus  which  sends  this  cell 
developing  into  a  regular  mul-ticellular  ball;  perhaps  the  beginning 
of  pressure  on  this  ball  as  its  expansion  brings  it  against  the 
protoplasmic  lining  of  the  embryo-sac,  is  the  stimulus  which  sets 
the  cotyledons  developing  at  their  definite  places,  which  places 
themselves  may  be  fixed  by  the  positions  of  least  pressure; 
perhaps  the  contact  of  these  growing  cotyledons  with  one  another 
is  the  stimulus  which  starts  the  development  of  the  plumule  be- 
tween them,  and  starts  also  the  extension  to  form  hypocotyl  and 
root  in  the  other  direction.  Maybe,  or  probably,  I  am  wrong  as 
to  the  details  of  these  stimuli,  but  if  it  is  not  these  it  is  some  others 
of  similar  sort;  and  hi  any  case  my  speculations  illustrate  the 
principle  of  the  matter.  The  idea  is  confirmed  by  the  fact  that 
there  is  one  case  of  growth  stimulation  of  whose  nature  we  are 
reasonably  certain.  The  reader  will  recall  that  the  stimulus  given 
by  the  fusion  of  the  second  nucleus  of  the  pollen-tube  with  the 
nucleus  of  the  embryo-sac  starts  the  development  of  the  endo- 
sperm (page  299) ;  and  this,  or  some  other  phase  of  fertilization, 
is  the  stimulus  which  starts  not  only  the  hardening  of  the  seed 
coats  and  the  development  of  other  typical  seed  features,  but  also 
the  many  large  processes  involved  in  the  formation  of  the  fruit. 
It  is  a  fact  that  ordinarily  neither  endosperm,  seed  coats,  nor  fruit, 
develop  unless  fertilization  is  effected, — an  arrangement  that  is 
obviously  adaptive,  since  without  fertilization  they  would  all  of 
them  be  useless,  and  a  wasteful  drain  on  the  plant.  This  kind  of 
"linking  up"  of  the  processes  together  through  the  connection  of 
stimuli  is  believed  to  be  representative  of  the  method  whereby 
the  development  of  plants,  and  animals  too,  is  kept  in  harmonious 
and  continuous  progress.  It  is  essentially  the  same  method 
as  that  by  which  the  parts  of  a  complicated  machine  are  kept 
working  effectively  together,  each  special  part  of  the  mechan- 
ism being  geared  or  connected  to  some  of  the  others  in  such  man- 
ner that  the  movement  of  the  mass  as  a  whole  compels  each  part  to 
perform  its  destined  office  at  just  the  right  moment  and  place. 


The  Orderly  Cycles  Pursued  in  Growth  357 

The  analogy,  indeed,  goes  a  long  way  farther,  for,  just  as  the 
accidental  loosening  or  breaking  of  some  connection  causes  the 
machine  to  work  irregularly,  or  even  causes  its  different  parts 
to  work  independently  of  one  another, — so  the  failure  of  the 
stimulus-connection  in  the  organism  may  release  some  parts  from 
the  regulatory  control  of  the  remainder  and  cause  them  to  work 
more  or  less  independently.  Such  is  without  doubt  the  explana- 
tion of  the  abnormal  or  monstrous  growths  presently  to  be  con- 
sidered. The  same  thing  is  well  known  in  the  animal  kingdom, 
where  tumors,  for  example,  are  known  to  be  growths  released  in 
some  way  from  the  regulatory  control  usually  exercised  by  their 
connection  with  the  rest  of  the  organism;  and  we  have  the  same 
thing  in  mental  phenomena,  for  dreams,  in  all  probability,  are 
simply  mental  processes  whose  correlation  is  temporarily  lost  in 
sleep,  while  insanity  is  the  same  thing  with  the  correlation  more 
or  less  completely  or  permanently  lost. 

While  the  embryo  is  thus  developing  from  the  egg-cell  and  the 
seed  from  the  ovule,  the  fruit  is  developing  from  the  ovary  and 
other  parts  of  the  flower;  and  this  fruit  aids  in  dissemination,  by 
the  methods  we  shall  later  consider.  During  dissemination,  and 
often  for  long  after,  the  seed  remains  in  a  resting  state,  with  its  vi- 
tality suspended.  In  most  seeds  this  resting  period  is  compulsory 
for  a  time,  at  least  for  several  weeks,  within  which  period  the  seed 
will  not  germinate  no  matter  how  favorable  the  conditions  that 
may  be  offered.  The  same  thing  is  true,  by  the  way,  of  winter 
buds,  bulbs,  and  some  other  plants,  though  it  is  interesting  to  note 
that  many  cultivated  plants,  notably  the  grains,  have  lost  the  rest- 
ing period,  and  will  germinate  as  soon  as  ripe,  even  sometimes  in 
the  seed  pod.  The  advantage  of  the  resting  period  to  the  plant  is 
sufficiently  plain:  it  gives  time  for  dissemination  and  it  prevents 
premature  germination,  such  as  might  happen  during  a  warm 
tune  in  winter,  resulting  in  the  destruction  of  the  embryo  by  the 
subsequent  frosts.  It  is  effected  and  controlled,  of  course,  by  the 
protoplasm,  which  uses  various  arrangements  for  the  purpose, — 


358 


The  Living  Plant 


sometimes  seed  coats  so  constituted  as  to  take  days  or  weeks  for 
water  to  penetrate  them,  sometimes  a  delay  in  the  development 
of  the  enzymes  needed  to  soften  the  endosperm,  sometimes  no 
doubt  in  yet  other  ways  that  are  still  undetermined. 

Thus  is  the  new  plant  developed  in  the  seed  prior  to  its  birth. 

2.  The  Growth  Cycle;  Germination. — This  is  a  very  distinct 
though  brief  stage.  When  the  resting  period  is  completed,  the 
seed  germinates  on  the  first  access  of  water  in  conjunction  with 
warmth.  The  water  is  absorbed  and  passed  on  to  the  embryo, 


. 


FIG.  136. — A  generalized  drawing  of  a  typical  case  of  germination,  from  the  dry  seed  to 
the  fully  grown  embryo.    The  controlling  factors  are  discussed  in  the  text. 

which  swells  powerfully,  and  thus  bursts  open  the  seed  coats. 
Immediately  the  root  grows  rapidly  out,  and,  no  matter  in  what 
position  the  seed  or  embryo  may  happen  to  he,  invariably  turns 
downward  under  the  stimulus  of  gravitation,  and,  develop- 
ing a  zone  of  anchoring  and  absorbing  hairs,  proceeds  to  grow 
straight  into  the  earth  (figure  136).  This  is  an  obvious  adapta- 
tion to  the  new  plant's  first  needs,  a  firm  anchorage  in  the  soil 
and  a  supply  of  water  therefrom.  When  the  root  is  thus  firmly 
anchored,  the  embryonic  stem,  under  the  stimulus  of  gravitation, 
begins  to  turn  upward,  and,  guided  by  other  stimuli,  works  the 
cotyledons  out  of  the  seed,  and  carries  them  upward,  where, 


The  Orderly  Cycles  Pursued  in  Growth  359 

responding  to  the  stimulus  of  light,  they  open  out,  turn  green, 
and  serve  as  the  first  foliage.  At  least  such  is  the  procedure  in  the 
most  typical  cases,  though  there  are  many  variations  in  detail, 
including  especially  a  great  many  cases  in  which  the  cotyledons 
remain  in  the  seed,  sending  up  only  the  plumule.  Meanwhile 
this  embryo  has  continued  to  absorb  water,  with  which  its  cells 
have  swelled  greatly;  and  its  stored  food  has  been  largely  con- 
verted to  new  wall  and  living  substance.  Finally  it  stands  up 
stiffly,  many  times  larger  than  at  first,  but  with  no  new  parts  and 
even  less  of  solid  substance.  It  is  now  all  ready  to  begin  its  in- 
dependent life. 

Thus  is  the  new  plant  born. 

3.  The  Growth  Cycle:  the  Seedling. — The  stages  of  develop- 
ment, while  distinct  in  the  main,  overlap  in  some  places.  Thus 
the  root  develops  considerably  in  germination,  and  early  in  this 
stage  begins  to  branch.  Its  new  cells  are  all  formed  at  a  definite 
growing  point,  whence  they  radiate  in  regular  lines  backward,  in- 
creasing in  size,  as  shown  very  clearly  in  our  earlier  figure  53,  and 
diagrammatically  in  a  later  figure,  139  D.  The  branches  of  roots 
start  always  from  the  fibrovascular  bundles  and  have  therefore 
to  break  or  dissolve  their  way  out  through  the  cortex  (figure  67) , 
a  method  which  seems  clumsy,  but  is  doubtless  the  best  that  the 
plant  can  do.  Their  places  of  origin  are  fixed  largely  by  exter- 
nal stimuli, — the  contact  of  greater  warmth,  moisture,  aeration, 
mineral  supply  and  the  like.  The  guiding  stimulus  of  their  sub- 
sequent growth  is  gravitation,  which  sends  them  radiately  out- 
ward in  directions  of  least  interference  with  one  another,  though 
they,  and  especially  the  later  branches,  are  swung  from  the 
geotropic  angles,  and  given  their  final  details  of  position,  by  ad- 
vantageous responses  to  various  minor  stimuli.  Thereafter,  so 
long  as  the  plant  lives,  these  roots  grow,  branch,  and  are  guided 
continuously  in  this  manner.  Meantime  the  embryonic  cells  be- 
tween the  cotyledons  become  active  and  push  up  a  cone  of  cells 
which  constitutes  the  first  bud.  As  this  bud  becomes  larger  the 


36o 


The  Living  Plant 


cell  divisions  become  more  active  at  definite  points  near  its  base, 
and  push  out  flat  projections  which  develop  and  grow  into  the 
leaves,  as  shown  by  our  accompanying  figure  137,  and  diagram- 
matically  by  figure  139,  C.  Unlike  new  roots,  the  leaves  have  their 
places  of  origin  determined  not  by  stimuli  from  without,  but  by 
internal  influences,  for  they  come  out  from  the  bud  in  accordance 


v 


FIG.  137. — A  bud  (of  Elodea,  a  water  plant),  in  surface  view  and  section,  showing  unusually 
clearly  the  mode  of  development  of  new  leaves.  (Copied  from  a  wall  diagram  by 
L.  Kny). 

with  definite  mathematical  systems,  as  we  have  considered  already 
under  phyllotaxy  (page  62).  In  their  early  stages,  and  while 
their  tissues  are  still  young,  the  leaves  are  flattened  closely  over 
one  another  into  the  conical  structure  we  commonly  call  a  bud; 
but  as  they  become  old  enough  to  be  useful  they  bend  outward 
and  ultimately  present  their  inner  faces  to  the  sun.  As  they  grow, 
their  blades  tend  to  take  horizontal  positions  under  guidance  of 
gravitation,  but  they  are  easily  swung  therefrom,  and  given 


The  Orderly  Cycles  Pursued  in  Growth  361 

final  direction,  by  the  stimulus  of  light,  to  which  they  set  their 
blades  at  right  angles.  As  the  leaves  develop,  embryonic  tissue 
in  their  axils  becomes  active,  and  develops  into  new  buds  pre- 
cisely like  the  first  bud  produced  by  the  embryo, — the  stimulus 
thereto  being  probably  the  pressure  ex- 
erted upon  them  by  the  developing  leaf. 
It  is  easy  to  see  an  advantage  in  this 
axillary  position  of  buds,  for  their  first 
need  is  abundance  of  food,  and  the  leaves 
are  the  source  of  supply.  From  these 
buds  grow  branches,  the  primary  direc- 
tions of  which  are  assumed  under  guid- 
ance of  gravitation,  whereby  they  are 
sent  radiately  out  into  positions  of  least 
interference  with  one  another,  although 
the  details  of  their  ultimate  positions  are 
fixed  by  a  variety  of  minor  influences, 
precisely  as  in  the  case  of  the  root- 
branches  above  mentioned. 

Such  is  the  complete  structure  of  a 
seedling,  of  which  a  typical  example  is 
here  represented  (figure  138). 

4.  The  Growth  Cycle:  the  Adult.— The 
seedling  continues  development  and  FIG.  138.— A  typical  seedling 

,,      ,.  .  ,         ,  ,        , .  .,  (of    a    Maple),   showing    the 

growth  for  a  considerable  tune  in  the  distinctive  parts,  excepting 
manner  just  described,  branching  con-  •JJ^SStt'coS 

tinuously  into  new  roots  and  stems,  and      ^d  fr°m  Gray's  structural 

Botany.) 

making  new  leaves.     Its  transition  to 

the  adult  condition  may  be  considered  as  marked  by  the  beginning 
of  reproduction,  even  though  the  plant  may  by  no  means  have 
reached  its  full  size.  Suddenly,  at  some  tune  in  the  plant's  growth, 
without  any  apparent  reason,  some  buds  begin  to  produce  flowers 
instead  of  more  leaves.  The  central  features  of  flowers  are  in 
reality  the  pollen  grains  and  the  embryo-sacs,  and  there  can  be 


362  The  Living  Plant 

little  doubt  that  it  is  the  beginning  of  the  formation  of  these  which 
gives  the  stimulus  to  the  formation  of  sepals,  petals,  stamens,  and 
pistils,  instead  of  ordinary  leaves.  But  it  is  not  yet  clear  what  it  is 
which  starts  this  formation  of  pollen  grains  and  embryo-sacs, 
though  it  must  result  in  part  from  some  outside  stimulus,  since 
plants  can  be  made  to  flower  much  sooner  by  making  the  external 
conditions  somewhat  harder.  The  flower,  once  formed,  secures 
pollination  or  cross  pollination  preparatory  to  fertilization,  as 
described  in  our  earlier  chapters,  and  is  followed  by  the  fruit 
which  aids  in  dissemination,  as  we  shall  consider  in  the  chapter 
that  follows.  But  with  the  flower,  indeed,  we  are  back  to  the 
fertilized  egg-cell  with  which  we  began,  and  thus  is  the  cycle 
completed. 

A  matter  of  very  much  interest  in  connection  with  the  growth 
of  plants  from  the  seedling  to  the  adult  concerns  the  changes  in 
their  tissues.  The  tissue  of  the  young  embryo  is  all  capable  of  cell 
division  (is  meristematic,  in  anatomical  language),  but  as  the 
embryo  germinates,  only  the  tip  of  the  root  and  the  first  bud,  to- 
gether with  a  thin  hollow  cylinder  of  cambium  connecting  them, 
remain  so,  while  all  the  remainder  of  the  cells  grow  large,  assume 
special  functions,  and  lose  their  power  of  division.  The  new 
growing  points  as  they  originate,  whether  on  stems  or  on  roots, 
establish  connections  with  this  cambium  cylinder  so  that  to- 
gether they  form  one  continuous  system,  in  which  all  of  the  grow- 
ing points  are  connected  with  one  another  by  hollow  cylinders  of 
cambium,  and  conversely,  the  cambium  cylinder  branches  into 
numerous  tapering  tubes  terminating  in  the  growing  points,  as 
our  diagrammatic  figure  illustrates  (figure  139).  Meantime  the 
cambium  grows  steadily  outward,  as  the  growing  points  grow 
steadily  onward,  each  forming  permanent  tissues  behind  them. 
This  separation  of  growth  and  permanent  tissues  makes  it  possible 
for  a  plant  to  go  on  growing  without  limit,  and  were  it  not  for  the 
restrictions  imposed  by  external  physical  conditions,  there  is  no 
reason  why  trees  should  not  be  immortal.  In  this  possession  of 


Longitudinal  section  through    ,/>.    Longitudinal section  throujh 


FIG.  139. — Generalized  drawings  illustrating  the  growth  system  of  the  plant. 
363 


364  The  Living* Plant 

continuously  working  embryonic  tissues,  plants  are  sharply  dis- 
tinct from  animals,  all  of  whose  tissues  sooner  or  later  become  of 
the  permanent  kind,  thus  limiting  their  further  growth. 

Although  the  most  typical  stems  possess  this  remarkable 
cambium  system,  there  are  others  which  lack  it.  In  these  the 
further  growth  takes  place  by  the  addition  of  new  fibrovascular 
bundles,  or  veins,  among  and  outside  of  the  old  ones,  so  that  the 


FIG.  140. — Cross  sections  of  stems  of  the  two  typical  kinds, — endogenous  with  scattered 
bundles  (the  Palm  on  the  left),  and  exogenous  with  the  bundles  in  rings  (the  Scotch 
Fir  on  the  right).  The  matter  is  further  explained  in  the  text. 

fully-grown  stem  is  composed  of  separated  bundles  scattered 
irregularly  through  the  ground  tissue,  as  well  seen  in  the  Corn,  or 
a  Palm  stem  (figure  140,  left),  or  any  plants  of  the  great  clas- 
sificatory  division  of  the  Monocotyledons.  Observation  alone,  of 
these  stems,  conveys  the  impression,  though  a  false  one,  that  the 
new  bundles  originate  inside  of  the  old  ones,  whence  they  have 
been  described  improperly  as  endogenous,  in  contrast  with  the 
exogenous  growth  from  cambium  (figure  140,  right).  The  growth 
of  exogenous  stems  involves  matters  of  much  interest,  as  figure  141 


The  Orderly  Cycles  Pursued  in  Growth  365 


will  help  to  illustrate.  Although 
these  stems  possess  separate  fibro- 
vascular  bundles  with  intervening 
plates  of  soft  tissue  at  the  start 
(compare  figure  73),  the  continuous 
growth  of  the  cambium,  which 
forms  new  duct  tissue  on  its  inner 
and  new  sieve  tissue  on  its  outer 
side,  gradually  fuses  the  bundles 
into  one  woody  mass,  although 
preserving,  more  or  less  perfectly, 
the  intervening  plates  of  tissue 
called  medullary  rays.  The  growth 
of  the  cambium,  however,  is  peri- 
odically checked  by  the  winter,  and 
the  contrast  between  the  small 
compact  autumn-formed  cells  and 
the  large  loose  tissue  of  the  spring 
(figure  139,  J3)  gives  rise  to  the 
familiar  phenomenon  of  the  annual 
rings,  which  appear  also,  though 
faintly  and  in  reverse  order,  and 
ultimately  crushed  to  unrecogniza- 
bility,  in  the  bark.  Most  of  these 
features  show  well  in  such  a  wood 
as  that  of  the  Oak,  where  the 
annual  rings,  and  even  the  sepa- 
rate ducts,  are  easily  visible  to  the 
eye,  while  the  medullary  rays  be- 
come the  broad  shining  plates 
which  give  distinction  and  value  to 
quartered  oak.  Meanwhile  corky 
waterproof  layers  are  forming  in 
the  outer  part  of  the  bark,  which 


year  old  nbrovascular  bundle,  01  a 
typical  stem,  the  Linden,  showing 
the  annual  rings  of  the  wood,  the 
cambium  cylinder,  and  the  annual 
rings  (less  prominent)  in  the  bark. 
Compare  with  a  single  bundle  in  fig- 
ure 139,  B.  (Copied  from  a  wall- 
chart  by  L.  Kny.) 


366  The  Living  Plant 

includes  everything  outside  of  the  cambium.  Though  the  bark, 
like  the  wood,  thus  increases  in  thickness  as  long  as  the  tree  lives, 
at  least  theoretically,  in  practice  it  weathers  away  about  as  fast 
on  the  surface  as  it  forms  inside.  It  will  now  be  clear  why  this 
exogenous  mode  of  growth  permits  the  indefinite  expansion  of 
woody  stems. 

In  close  relation  to  the  age  at  which  flower  buds  first  appear  is 
the  length  of  life  of  the  plant.  When  plants  come  to  flower  the 
season  they  germinate,  all  of  the  food  they  can  make  is  thrown 
into  their  seeds,  and  the  soft  stems  then  die,  root  and  branch: 
such  plants  are  annuals.  Other  kinds,  however,  make  only  leaf 
buds  the  first  season,  and  store  up  food  in  some  underground  part 
to  which  they  die  down;  then  this  food  is  made  use  of  in  forming 
new  stems,  flowers  and  seeds  the  next  season,  after  which  the 
plants  perish  completely:  such  plants  are  biennials.  Yet  other 
kinds,  in  the  second  season,  instead  of  throwing  all  of  their  food 
into  seeds,  store  a  part  underground,  die  down  thereto  and  then 
send  up  a  new  flowering  stem  the  next  season,  and  so  on  year  after 
year:  such  plants  are  herbaceous  perennials,  which  include  so 
many  of  the  favorites  of  our  gardens.  Finally,  there  are  many 
others  which  do  not  die  down  to  the  ground  at  all,  but  harden 
their  stems  to  wood,  and  thus  can  stand  upright  over  winter. 
Thereafter,  each  season's  growth,  whether  in  length  or  in  thick- 
ness, is  built  upon  that  of  the  preceding,  and  the  structure  thus 
grows  both  hi  length  and  in  thickness  as  long  as  it  lives:  such 
plants  are  woody  perennials,  which  are  principally  shrubs  and 
trees.  Since  this  method  admits  of  indefinite  increase  in  size,  and 
since,  moreover,  it  involves  a  constant  rejuvenescence  of  the 
protoplasm  (the  significance  of  which  is  discussed  earlier,  on 
page  162),  it  is  obvious  that  trees  have  no  limit  set  to  their  growth 
by  internal  factors,  but  their  maximum  size  is  imposed  by  the 
action  of  extrinsic  causes, — such  for  example  as  the  increasing 
difficulty  of  conducting  sufficient  water  supply  through  the 
greatly  lengthening  stems.  Thus,  with  increasing  size  it  becomes 


The  Orderly  Cycles  Pursued  in  Growth  367 

more  difficult  for  them  to  transfer  a  sufficiency  of  water  and  min- 
erals to  their  more  distant  parts,  whose  vitality  is  thus  checked. 
At  the  same  time  the  increasing  exposure  of  parts  to  the  winds, 
and  the  greater  leverage  thus  given,  leads  to  breakage,  and  hence 
the  admission  of  rot-producing  fungi,  which  sooner  or  later  bring 
the  loftiest  tree  to  the  ground.  Thus  trees,  though  they  grow  very 
large,  never  really  stop  growing  in  size;  and,  moreover,  they  never 
grow  old  in  the  sense  that  animals  do,  but  come  to  their  end  while 
their  individual  parts  are  still  in  full  vigor. 

Such  is  the  cycle  of  growth  in  the  most  highly  organized  plants, 
and  very  different  it  is,  as  the  reader  will  have  noticed,  from  the 
cycle  displayed  in  the  highest  of  animals.  For  animals  construct 
but  a  single  set  of  organs,  which  last  without  renewal  through  life; 
and  when  these  have  each  grown  to  full  size  the  growth  of  the 
individual  is  stopped,  though  it  may  live  for  a  very  long  period 
thereafter.  Inside  of  these  organs  the  protoplasm  goes  on  working 
without  chance  for  rejuvenescence,  and  therefore  gradually  wears 
out  and  dies,  thus  fixing  an  internal  limit  to  the  length  of  the 
animal's  life. 

Through  such  a  complex  though  orderly  cycle  do  the  most 
highly  organized  plants  all  swing  in  the  course  of  their  develop- 
ment and  growth.  In  tropical  climates  the  cycle  is  accomplished 
without  pause,  except  for  a  brief  time  during  dissemination,  but 
in  temperate  regions  the  continuity  is  rudely  disturbed  every 
year  by  the  advent  of  winter,  to  which  all  vegetation  must  in 
some  way  make  adjustment.  One  could  hardly  believe,  a  priori, 
that  plants  could  accommodate  themselves  to  ranges  of  tempera- 
ture from  forty  degrees  below  zero  to  a  hundred  and  twenty 
above  it; — yet  such  is  the  fact.  This  adjustment  of  vegetation  to 
winter  introduces  a  secondary  seasonal  cycle,  which  has  the  four 
following  stages: 

The  Winter  is  the  season  of  dormance,  in  which  vitality  is  sus- 
pended. The  protoplasm,  giving  up  the  most  of  its  water,  ceases 
to  move,  becomes  hard,  reduces  all  activities  to  a  minimum,  and 


368  The  Living  Plant 

goes  into  a  resting  state.  Its  condition  in  the  buds,  the  cambium, 
and  the  other  living  parts  then  approximates  to  that  of  the  seed. 
This  is  the  season  of  gray  and  brown  colors,  which  are  distinctive 
of  dried  tissues  and  of  the  non-conspicuous  protective  bark  and 
bud  scales. 

The  Spring  is  the  season  of  unfolding,  when  the  plant  absorbs 
water,  the  sap  rises,  and  the  protoplasm  awakens  to  a  new  and 
exuberant  life.  Then  all  of  the  structures  developed  in  the  buds 
the  preceding  season  enlarge  rapidly  through  their  grand  period, 
and  unfold  to  soft-textured  foliage  and  flowers,  transforming  the 
whole  face  of  Nature.  This  too  is  the  principal  season  of  fertiliza- 
tion, and  of  germination,  and  of  new  life  in  various  forms.  It  is 
the  season  of  delicate  colors,  for  not  only  is  it  the  tune  of  most 
flowers,  and  of  the  softest  foliage  green,  but  much  of  the  young 
vegetation  is  overspread  by  a  blush  of  rosy  red,  which  perhaps  is 
the  protective  and  warming  screen  to  the  much  exposed  proto- 
plasm before  the  chlorophyll  is  fully  made. 

The  Summer  is  the  season  of  accumulation.  The  green  leaves, 
in  the  full  vigor  and  strength  of  maturity,  are  engaged  in  the 
making  of  food,  which  passes  steadily  away  to  the  places  of 
storage  or  use,  providing  for  current  needs  and  preparing  a  new 
store  for  the  future.  It  is  the  time  of  development  of  embryos,  and 
formation  of  fruits.  It  is  the  season  of  greenness,  the  typical 
time  of  vegetation,  the  state  that  is  permanent  in  the  tropics. 

The  Autumn  is  the  season  of  fruition, — the  tune  of  ripening 
which  is  always  a  preparation  for  the  future.  The  tissues  are 
strengthened  and  hardened;  the  parts  to  come  out  the  next  spring 
are  completed  in  the  buds  and  enwrapped  with  protective  covers; 
the  fruits  are  brought  to  perfection  and  made  ready  for  their  func- 
tion of  dissemination;  the  leaves  are  emptied  of  their  valuable 
matters  and  made  ready  for  their  annual  fall;  the  protoplasm 
throughout  the  plant  gradually  yields  up  its  water  and  assumes  its 
resting  condition.  It  is  the  season  of  brilliant  colors,  red,  purple 
and  yellow,  a  part  (those  of  fruits)  serving  definite  uses,  but  the 


The  Orderly  Cycles  Pursued  in  Growth  369 

most  of  them  (the  autumn  foliage),  a  chemical  accident,  though 
one  with  the  happiest  consequences  to  the  pleasure  of  man.  Then 
the  winter  comes  again  and  the  seasonal  cycle  is  closed. 

There  remains  yet  one  other  aspect  of  growth,  and  that  of  no 
little  importance, — namely,  the  remarkable  results  that  ensue 
from  its  disturbance.  All  growth  when  left  undisturbed  tends  of 
itself  to  produce  symmetrical  structures.  In  evidence  thereof 
one  has  only  to  recall  the  superb  symmetry  of  an  Elm  or  a  Maple 
when  growing  alone  in  a  meadow,  or  the  perfection  of  conical 
form  in  a  Fir  tree  which  springs  up  in  some  field  that  is  abandoned, 
or  the  regularity  in  arrangement  of  leaves  which  develop  within 
the  protection  of  a  bud.  And  the  same  thing  can  be  shown  very 
beautifully  by  experiment.  Accordingly  when  a  plant  deviates 
from  symmetry  it  is  always  because  of  disturbing  influences, 
of  which  there  are  some  four  classes. 

Disturbance  of  Growth  by  Accidents. — These  are  many  and  so 
obvious  as  hardly  to  need  comment,  including  overcrowding  by 
other  plants,  breaking  of  branches  by  wind  or  ice,  destruction  of 
parts  by  animals  or  Fungi,  scalding  of  newly  exposed  bark  by  the 
sun,  drying  back  of  parts  through  failure  in  water  supply,  poison- 
ing by  bad  gases,  and  many  others  of  various  sorts.  And  it  is 
important  to  observe  that  the  destruction  of  parts  of  a  plant  by 
any  of  these  agencies  is  followed  as  a  rule  by  an  effort  at  replace- 
ment and  the  restoration  of  the  symmetry,  as  can  be  seen  in  trees 
which  have  lost  some  of  their  branches. 

Disturbance  of  Growth  by  Adaptive  Adjustment  to  the  Surround- 
ings.— This  subject  received  full  treatment  in  an  earlier  chapter, 
where  it  was  shown  that  individual  plants  can  alter  greatly  the 
details  of  their  form,  size,  or  structure,  in  adjusting  themselves  to 
take  advantage  of  the  best  conditions  of  their  immediate  en- 
vironments. It  is  most  conspicuous  in  connection  with  adjust- 
ment to  light,  towards  which  a  plant  will  often  bend  its  entire 
structure;  or  in  connection  with  adjustment  to  moisture,  towards 
which  an  entire  root  system  will  often  extend  in  a  very  unsym- 


370  The  Living  Plant 

metrical  fashion.     Such  disturbances  of  symmetry  are  wholly 
natural  and  healthful,  unlike  the  cases  which  follow. 

Disturbance  of  Growth  by  Parasitic  Stimulation. — The  parasites 
of  plants  are  of  two  general  classes,  Insects  and  other  plants, 
mostly  of  the  simple  kinds  called  Fungi.  Everybody  knows 
the  structures  called  Galls,  especially  common  and  typical  upon 
Oak  leaves,  where  they  appear  as  rounded,  almost  nut-like,  often 
hairy,  sometimes  red  swellings  which,  when  opened  reveal  al- 
ways the  presence  of  a  living  insect  larva.  There  are  hundreds 
of  kinds  of  these  galls,  very  different  from  each  other  but  each 
kind  so  distinctive  that  an  expert  can  distinguish  them  easily,  and 
even  identify  the  insect  which  made  them.  Other  galls  are  almost 
hair-like,  others  are  globular  swellings  of  very  slender  stems,  while 
yet  others  include  the  terminal  buds,  and  involve  the  leaves  in 
a  way  to  produce  those  compact  rose-shaped  structures  often 
called  willow-roses.  They  are  all  made  in  substantially  the  same 
manner;  an  insect  lays  its  egg  in  the  growing  soft  tissue,  and  the 
developing  insect  causes  the  plant  tissue  around  it  to  form  such  a 
structure,  and  to  lay  up  such  contents,  as  will  provide  both  a  safe 
home  and  a  sufficient  food  supply  for  the  larva  until  its  maturity, 
when  it  makes  its  way  out  and  away.  But  just  how  the  result  is 
effected  by  the  insect  is  not  at  all  certain,  whether  by  mechanical 
movements  or  chemical  secretion.  Nor  is  it  yet  certain  just  what 
the  plant's  attitude  is  towards  the  gall.  It  can  hardly  be  true 
that  the  plant  derives  benefit  from  the  excretions  of  the  insect, 
since  the  original  substance  to  make  those  excretions  is  mostly 
supplied  by  the  plant  itself.  It  seems  much  more  probable  that 
the  plant  is  passively  affected  by  the  insect,  which  has  discovered, 
so  to  speak,  just  the  chemical  substance  or  the  developmental 
stimulus  which  happens  so  to  fit  some  peculiarity  of  the  metab- 
olism of  the  complicated  protoplasm  as  to  stimulate  it  to  the 
formation  of  structures  and  substances  adaptive  to  the  uses  of  the 
insect.  Theoretically,  man  ought  to  be  able  to  affect  plants  in 
analogous  ways,  and  it  is  not  unlikely  that  the  horticulture  of  the 


The  Orderly  Cycles  Pursued  in  Growth  371 


future  may  embrace  wonderful  new  vegetables  or  fruits  produced 
by  the  injection  of  chemical  substances  into  the  young  growing 
tissues  of  ordinary  plants. 

Of  somewhat  analogous  nature  are  the  abnormal  growths  pro- 
duced by  the  presence  of  parasitic  plants.  A  typical  case  is  found 
in  those  remarkable  dense  growths 
of  many  slender  upright  twigs, 
found  often  on  Spruce  trees  and 
commonly  mistaken  for  nests  of 
big  birds.  They  are  generally 
known  as  Witches  Brooms,  and 
are  caused  by  the  presence  of  a 
Fungus  which  sends  its  nutritive 
threads  into  the  young  growing 
branch,  and  seems  to  produce  a 
paralysis  of  the  delicately-bal- 
anced controlling  power  of 
growth.  As  a  result  all  the  buds 
in  that  region  proceed  to  develop, 
though  ordinarily  they  would 
mostly  be  suppressed,  and  each 
grows  for  itself  without  any  ge- 
otropic  correlation  with  its  neigh- 
bors. Of  precisely  similar  nature 
are  those  spiral-radiate-saucer  growths  on  the  roots  of  tropical 
trees,  often  sold  to  tourists  as  curiosities  under  the  name  of 
"wooden  flowers"  (figure  142).  Unlike  the  case  of  the  galls, 
there  is  no  obvious  advantage  to  the  parasite  in  these  methods 
of  growth  of  its  host,  and  the  result  seems  purely  incidental 
to  the  paralysis  of  the  regulatory  power, — the  direction  that 
anarchy  takes,  as  it  were,  when  the  hand  of  the  law  is  re- 
moved. We  see  something  of  the  very  same  sort  in  the  animal 
world  and  especially  in  mankind,  in  tumors  and  other  abnormal 
growths,  which  are  likewise  a  continuation  of  growth  without 


FIG.  142. — A  "wooden  flower"  from 
Guatemala,  one-third  the  natural  size. 
It  is  explained  in  the  text. 


372  The  Living  Plant 

control.  From  such  elaborate  structures  as  the  Witches  Brooms 
there  are  all  grades  downward  to  those  so  inconspicuous  as  hardly 
to  attract  notice,  including  some  very  simple  kinds  of  excrescences. 

Results  very  similar  to  these  may  be  caused  by  external  me- 
chanical injuries.  Thus,  when  tree  trunks  are  injured  in  the  cam- 
bium, this  also  loses  its  regularity  of  growth,  and  becomes  thrown 
into  various  contortions,  producing  gnarly  fibrous  growths  which 
often  appear  as  large  burls  on  the  trunks.  Moreover,  the  injured 
cambium  at  times  attempts  to  put  out  adventitious  buds,  which, 
forming  in  large  numbers  but  without  proper  control,  just  about 
keep  pace  with  the  expansion  of  the  trunk.  The  resultant  masses 
of  wood  show  the  characteristic  rings  of  buried  branches,  often  in 
patterns  displaying  great  beauty,  of  which  the  Birdseye  Maple 
affords  a  conspicuous  example. 

Disturbance  of  growth  by  Internal  Causes. — In  addition  to  the 
external  and  visible  causes  which  throw  growth  into  confusion, 
there  are  others  which  appear  to  be  internal.  One  of  the  simplest 
cases  is  that  in  which  the  correlation  between  the  different  parts 
of  a  plant  becomes  weakened  or  lost.  This  correlation  is  beauti- 
fully shown  in  the  familiar  fact  that  if  the  young  tip  of  the  main 
stem  of  a  Spruce  or  a  Fir  tree  be  cut  away,  one  of  the  nearest 
branches  will  grow  up  and  take  its  place,  although,  had  the  tip 
remained,  that  branch  would  have  grown  like  its  neighbors 
horizontally  outwards.  It  is  indeed  this  correlation  of  the  geot- 
ropism  of  the  branches  which  makes  possible  the  symmetrically 
radiate  shape  of  a  tree,  each  branch  as  it  grows  assuming  the 
correct  geotropic  angle  to  form  either  the  cone  or  the  ball  of 
foliage  as  the  case  may  be.  Now  in  old  trees  this  correlation  is 
sometimes  lost,  perhaps  through  the  interruption  of  the  pro- 
toplasmic connections  along  the  stem,  and  then  each  new  branch 
grows  upward  precisely  as  if  it  were  the  only  main  stem,  as  our 
accompanying  figure  illustrates  (figure  143).  Obviously  such 
cases  are  related  in  method  with  the  Witches  Brooms  and  the 
like,  earlier  mentioned.  Somewhat  the  same  thing  occurs  in 


The  Orderly  Cycles  Pursued  in  Growth  373 


the  case  of  branches,  or  sprouts,  that  spring  anew  from  old 
trunks. 

More  profound  disturbances,  also  of  internal  origin,  result  in 
the  formation  of  monstrosities,  or  in  common  language,  " freaks." 
They  are  really  rather  common,  and  at- 
tract much  notice  because  of  their  oddity. 
In  general  they  may  be  distinguished 
from  the  effects  of  fungus  or  insect  action 
by  the  fact  that  although  they  look  odd 
they  also  look  healthy.  Very  typical  are 
those  called  fasciations,  which  arise  in 
this  wise,  that,  from  causes  unknown, 
some  terminal  bud,  instead  of  develop- 
ing as  a  single  cylindrical  structure,  be- 
comes partially  split  into  a  number  of 
points,  which  usually  spread  out  like  a 
fan,  and  produce  a  flattened  or  corru-  FIG.  143.— An  old  Apple  tree, 

o-nfprl     «5fpTYi     with     Trmnv    littlp    fprmirml        in    whicn  the  geotropic  cor- 

gatea  stem  witn  many  i  .rminai     reiation  of  the  branches  has 

points.     A  remarkably  fine  example  of  a      Jpeen  los*'  leaving  each  bud 

free  to  develop  as  if  it  were 

faSCiated   Pineapple  is  Shown    by  the   aC-        the  only  one.     (Traced  from 
.  ..  ,f*  .  .^  ,        a  photograph.) 

companying    picture    (ngure    144),    and 

most  people  have  seen  fasciations  in  Asparagus,  Hyacinths,  or 
other  strong-growing  plants.  Fasciations  are  also  the  basis  of 
the  crested  forms  of  Cactus  and  other  plants,  and  give  the 
"  cockscomb  "  to  the  plant  of  that  name,  in  which,  as  in  some  other 
cases,  the  condition  is  hereditary.  Fasciations  can  also  be  pro- 
duced, by  the  way,  by  external  injury,  such  as  the  bites  of  some 
insects,  though  when  produced  in  such  manner  they  are  not  hered- 
itary. They  are  of  all  degrees  of  complexity,  down  to  a  simple 
forking  of  the  growing  point,  which  may  sometimes  result  in  the 
formation  of  double  fruits,  though  these  are  more  often  the  result 
of  the  fusion  of  two  buds  in  a  sort  of  natural  grafting.  It  is 
obvious  that  such  fasciations  come  very  close  to  the  condition 
which  originates  the  Birdseye  Maple,  or  rather  that  the  latter 


374  The  Living  Plant 

in  reality  is  a  kind  of  fasciation.  It  is  perfectly  impossible  to 
draw  any  sharp  line  between  these  different  forms  of  clustered 
abnormal  growths,  or  between  external  and  internal  causes  of 
their  formation. 

Somewhat  similar  is  the  origin  of  twisted  stems  or  torsions. 
These  occur  in  small  herbs,  but  are  often  seen  to  perfection  in 
dead  standing  trees,  or  even  in  the  logs  of  fence  rails;  but  here  the 


FIG.  144. — A  fasciated  Pineapple,  resulting  from  causes  explained  in  the  text. 

process  is  hardly  abnormal,  since,  as  seems  likely,  the  twisting  of 
the  cambium  cylinder,  to  which  it  is  due,  is  a  result  of  normal 
growth  processes  in  the  plant. 

A  second  common  form  of  monstrosity  is  that  known  as  pro- 
liferation. Sometimes  the  stem  of  a  pear,  or  a  strawberry  (fig- 
ure 145)  grows  on  beyond  the  fruit,  producing  there  a  cluster  of 
leaves.  In  one  way  these  cases  are  easy  to  understand,  for  they 


The  Orderly  Cycles  Pursued  in  Growth  375 

are  simply  instances  wherein  the  stem,  which  ordinarily  ceases  to 
grow  in  a  flower  bud,  keeps  on  growing  just  as  it  does  in  the  leaf 
buds,  though  why  it  should  do  so  is  not  as  yet  known.  We  have 
a  partial  case  of  the  same  thing  in  the  navel  orange,  in  which  the 
receptacle  or  stem  grows  part  way  up  through 
the  fruit  and  makes  there  a  second  series  of 
carpels,  which  constitute  the  secondary  orange 
within  the  navel;  and  the  same  thing  carried 
farther  in  apples  sometimes  gives  us  a  two- 
storied  fruit. 

As  to  other  monstrosities  they  are  legion,  — 
enough,  indeed,  so  that  their  mere  synoptical 
description  suffices  to  fill  large  volumes  devoted 

to   the  subject.       Flowers  double  profusely; 

'  .  .    ;  "  FIG.   145.-  A    straw- 

leaves  instead  of  their  characteristic  flatness     berry,  in  which  the 

exhibit  often  the  form  of  a  pitcher  or  cup;      SS 


pistils  become  open  leaves,  exposing  the  ovules,      ^J0^^^  Jco°  ied 
which  themselves  become  altered  at  tunes  to      from  Masters'  vege- 

..,      a    ,  ,  ,  ,  table  Teratology.) 

tiny  leaflets;  apples  and  cucumbers  produce 
leaves  on  the  sides  of  their  fruits;  flowers  become  green,  and 
bracts  of  the  stem  assume  the  colors  of  flowers;  and  so  many 
other  alterations  of  form,  color,  size  and  regularity  occur  that 
it  sometimes  seems  as  if  every  deviation  from  normality  struc- 
turally possible  in  any  and  every  part  of  the  plant  became  some- 
time or  other  actually  realized  in  fact.  Some  of  these  mon- 
strosities are  hereditary,  though  mostly  they  are  not,  and  many 
of  them  could  be  propagated  by  grafting  if  it  were  thought 
worth  while.  It  is  evident  that  they  merge  without  break  over 
into  those  extreme  variations  which  are  called,  horticulturally, 
sports. 

Monstrosities  are  sometimes  reversions  to  an  ancestral  condi- 
tion, and  formerly  they  were  thought  always  to  be  so.  Hence 
they  were  supposed  to  throw  light  upon  evolution  and  classifica- 
tion, an  idea  embodied  in  Goethe's  saying  that  "by  her  mistakes 


376 


The  Living  Plant 


Nature  betrays  her  secrets."  But  so  many  monstrosities  are 
known  which  cannot  be  interpreted  as  reversions  that  we  must 
consider  them  rather  as  results  of  disturbance  of  the  growth 
processes,  though  we  have  no  idea  as  to  the  ultimate  causes. 
They  can  mostly  be  interpreted  in  terms  of  failure  of  action  on 
the  part  of  the  suitable  stimuli.  Thus,  in  green  roses,  the  stimuli 


FIG.  146.— Typical  examples  of  water-rolled  weed  balls,  photographed  about  two-fifths 
the  natural  size  (the  squares  of  the  screen  are  each  one  centimeter).  The  largest  is 
composed  of  various  Algae;  the  next  in  size,  of  Spruce  needles;  the  njxt,  of  Pipe-wort; 
the  oval  one,  of  hair;  while  the  composition  of  the  fifth  is  uncertain. 

which  started  the  formation  of  a  flower  bud  instead  of  a  leaf  bud 
worked  properly,  but  those  which  controlled  its  farther  develop- 
ment did  not.  But  as  to  the  causes  of  such  failure  of  stimuli  we 
have  no  information. 

In  connection  with  plant-structures  of  odd  mode  of  growth,  we 
must  take  note  of  one  having  a  very  different  character.     On 


The  Orderly  Cycles  Pursued  in  Growth  377 

some  sandy  shores  of  the  sea,  or  of  freshwater  lakes,  the  visitor 
sometimes  finds  balls  of  vegetable  matter  a  few  inches  through,  of 
a  roundness  and  symmetry  wholly  surprising.  Naturalists  once 
thought  them  a  species  of  seaweed,  while  peasants  and  doctors 
ascribed  to  them  medicinal  virtues.  In  reality  they  are  nothing 
other  than  masses  of  the  fibrous  parts  of  half-decayed  plants, 
matted,  compacted  and  rounded  by  the  gently-rolling  action  of 
the  underwater  parts  of  waves  acting  over  smooth  sandy  bottoms. 
They  are  made  up  of  the  most  diverse  materials, — plant-fibers, 
fine  seaweeds,  needles  of  pines,  spruces  and  hemlocks,  and  even 
such  adventitious  materials  as  shavings  and  hair.  A  number 
of  different  kinds  are  well  shown  on  the  accompanying  plate 
(figure  146).  These  are  not  the  only  kinds  of  vegetable  balls 
that  are  known;  others,  sometimes  called  bezoars,  are  formed  by 
the  rolling  and  matting  of  indigestible  fibers  in  the  stomachs  of 
cattle.  Somewhat  analogous  is  the  curious  algal  paper,  sometimes 
formed  by  the  drying  of  continuous  masses  or  sheets  of  matted 
Algae  left  by  the  falling  water  of  lakes.  And  doubtless  there  are 
other  structures  also,  which  appear  to  be  products  of  some  special 
mode  of  growth  while  in  reality  they  are  merely  a  result  of  the 
play  of  natural  forces. 


CHAPTER  XV 

THE    MANY    REMARKABLE    ARRANGEMENTS    BY 
WHICH  PLANTS  SECURE  CHANGE  OF  LOCATION 

Dissemination;  Fruits 

NE  of  the  most  obvious  and  consequential  of  the  facts 
about  the  typical  plants  is  their  sedentary  habit; — 
they  are  rooted  immovably  in  one  spot.  Yet  all  of  the 
kinds  are  able  at  some  stage  in  their  lives  to  change  their 
locations,  though  the  methods  whereby  this  is  done  are  most  di- 
verse, as  the  following  pages  will  abundantly  demonstrate. 

We  make  sure,  first  of  all,  why  a  change  of  location  is  needed. 
To  take  the  most  obvious  reason,  it  is  evident  that  if  all  of  the 
seeds  that  any  plant  ripens  were  to  fall  direct  to  the  ground  and 
germinate  there,  a  jungle  would  result  so  dense  that  few,  or  per- 
haps none,  of  the  plants  could  survive.  A  power  to  spread  from 
their  parents  is  therefore  essential  in  order  that  individuals  may 
find  space  in  which  to  develop.  But  there  are  reasons,  as  well,  of 
a  secondary  sort.  Thus,  any  kind  of  plant,  whether  because  it 
exhausts  from  the  soil  some  material  it  needs  in  its  growth,  or 
because  it  imparts  to  the  earth  some  excretion  injurious  to  itself, 
cannot  grow  a  very  long  time  in  a  single  location  without  deterio- 
ration of  vigor.  Again,  in  an  ever-changing  world,  it  is  an  ad- 
vantage to  any  species  if  it  can  leave  a  situation  becoming  less 
favorable  for  its  life  and  migrate  to  some  other  that  is  becoming 
more  favorable.  Furthermore,  it  seems  true  of  plants  as  of  men 
that  an  occasional  change  to  different  surroundings  acts  stimu- 
latingly  upon  health  and  adaptability,  and  therefore  is  distinctly 

378 


How  Plants  Secure  Change  of  Location  379 

advantageous  in  the  struggle  for  existence.  And  other  reasons 
exist,  of  lesser  weight,  which  combine  with  those  given  to  explain 
both  the  need  and  the  value  of  a  change  of  location. 

The  methods  whereby  plants  secure  this  change  of  location  are 
many  and  various,  but  fall  somewhat  naturally  under  these 
divisions : — 

1.  Independent  Locomotion 

2.  Extension  through  Growth 

3.  Projection  by  Elastic  Machinery 

4.  Waftage  by  Winds 

5.  Flotage  upon  Water 

6.  Carriage  by  Animals 

The  roll  of  these  methods  will  recall  to  the  reader  our  dis- 
cussion of  the  use  of  the  very  same  ones  in  connection  with  cross 
pollination.  They  are,  in  fact,  substantially  the  same,  as  would  be 
inferred  from  the  similarity  of  the  problems  presented  to  the 
plant  in  the  two  cases.  The  chief  differences  are  connected  with 
the  greater  difficulties  of  cross  pollination  (for  here  a  definite 
goal  as  well  as  a  definite  starting  place  is  imposed),  and  with  the 
extreme  fineness  and  lightness  of  pollen,  which  makes  its  pro- 
pulsion from  plant  to  plant  impracticable.  But  the  identity  of 
method  in  the  two  processes  should  not  be  permitted  to  create 
any  confusion  between  them  in  the  mind  of  the  reader,  who 
should  keep  very  clearly  hi  mind  the  totally  different  object  in 
the  two  cases. 

1.  Independent  Locomotion. — Although  none  of  the  higher,  or 
familiar,  plants  possess  this  power,  it  is  well  developed  in  the  sim- 
pler kinds  which  lack  the  firm  cellulose  skeleton;  and  the  method 
thereof  is  precisely  the  same  as  is  used  by  the  simpler  animals. 
Thus,  some  kinds  creep,  as  in  the  case  of  the  Slime-molds  (or 
Myxomycetes),  with  which  the  reader  has  already  made  acquaint- 
ance in  the  chapter  on  Protoplasm;  these  opaque- white  gelatinous 
masses  are  sometimes  seen  in  damp  places, — on  decaying  wood, 
wet  earth,  or  neglected  flower  pots, — where  they  creep  about, 


380  The  Living  Plant 

though  slowly,  by  the  simple  device  of  directing  their  protoplasmic 
streaming  in  one  constant  direction,  precisely  as  is  the  habit  of  the 
much  smaller  Amceba  among  animals  (figure  147).  Other  plants, 
again,  or  their  reproductive  spores,  can  swim  freely  in  water,  in  a 
manner  so  like  that  of  animals  that 
they  are  known  as  " animal  spores,"  or 
zoospores;  and  these  are  very  abundant 
and  characteristic  in  the  Algae  or  Sea- 
weeds (figure  94).  The  motion  is  ef- 
fected either  by  the  action  of  innumer- 
able cilia,  tiny  hairs  which  all  in  unison 

FIG.    147.— An    Amoeba,    greatly    ,  . 

magnified;  a  creeping  organism  beat  the  water  more  strongly  in  one 
direction  than  the  other,  or  else  by 

flagellae,  which  are  structures  suggestive  of  tails,  except  that 
instead  of  pushing  the  spore,  they  pull  it  behind  them  by  an 
action  the  reverse  of  the  one  used  in  the  tail  of  a  fish.  These 
movements  of  flagellae  and  cilia,  by  the  way,  depend  upon  the 
power  of  contractility  in  protoplasm,  a  form  of  the  motility 
which  has  already  been  described  as  one  of  the  physiological 
properties  of  that  substance;  and  this  same  contractility,  also, 
is  the  basis  of  the  muscular  mechanism  of  the  higher  animals. 
Other  kinds  of  water  plants,  notably  the  Blue-green  Algae,  make 
use  of  vibration  of  their  rod-like  bodies,  securing  their  movement, 
I  suppose,  in  essentially  the  same  way  that  a  piece  of  flexible  steel 
is  shot  through  the  air  after  having  been  bent  between  thumb 
and  forefinger  and  then  quickly  released.  Other  kinds  push  out 
protoplasmic  threads,  which  work  against  the  bottom,  as  is  the 
way  with  the  Diatoms, — those  tiny  plants  whose  wonderfully- 
sculptured  shells  are  the  favorites  of  every  happy  possessor  of  a 
first  microscope.  In  all  of  these  modes  of  locomotion,  the  re- 
semblance to  animals  is  not  accidental,  but  a  persistence  from  an 
ancient  condition  in  which  the  two  kingdoms  were  one. 

2.  Extension  through  Growth. — The  stems  of  the  higher  plants, 
as  the  reader  will  recall,  are  usually  so  made  that  elongated  inter- 


How  Plants  Secure  Change  of  Location          381 

nodes  separate  the  bud-bearing  nodes,  which,  moreover,  in  most 
soft-textured  plants,  can  readily  strike  root.  It  is  plain  that  if 
stems  are  sent  out  horizontally  in  such  manner  as  to  touch  the 
ground,  the  nodes  at  their  tips  may  strike  root  and  send  up  new 
shoots,  thus  originating  new  plants  at  some  distance  from  the 
parent,  from  which  they  will  later  be  cut  loose  by  the  death  of  the 
intermediate  stem.  Plants  have  not  been  slow  to-  take  advantage 
of  the  possibilities  of  this  method.  Everybody  knows  the  typical 
case  of  the  Strawberry,  with  its  long  slender  runners  which  bear 
tiny  plants  at  their  tips;  and  the  same  thing  is  found  in  the  House- 
leeks,  and  others  too  many  to  mention.  Some  plants  send  the 
stems  underground,  after  the  manner  of  roots,  and  form  new 
plants,  called  suckers,  at  places  not  possible  to  predict;  and  this 
makes  them  hard  to  exterminate,  as  in  the  case  of  the  Yarrow,  and 
some  other  weeds  of  pertinacious  character.  Suckers,  by  the  way, 
spring  also  from  roots,  some  kinds  of  which  can  make  buds,  es- 
pecially when  injured;  and  this  is  the  way  with  some  fruit  trees, 
like  Apples.  In  a  similar  manner,  the  horizontally-radiating 
underground  equivalents  of  roots,  the  mycelial  threads,  of  some 
Mushrooms,  send  up  the  new  Mushrooms  at  so  regular  a  distance 
from  the  parent  as  to  form  a  conspicuous  ring,  whose  name  "Fairy 
Ring,"  implies  an  ancient  belief  as  to  its  origin,  (figure  148). 
Again,  among  the  more  familiar  plants,  there  are  shrubs,  of  which 
our  Briars  and  Blackberries  are  examples,  with  stems  so  slender  as 
to  curve  over  and  bring  their  tips  to  the  ground,  where  they  take 
root  and  produce  new  plants,  known  as  stolons',  and  these  con- 
necting stems  for  a  time  form  a  trap  for  the  feet  of  the  unwary, 
giving  name  to  the  various  shrubs  called  Hobble-bush.  The  Walk- 
ing Fern  gives  another  example  of  this  method. 

There  are  plants,  however,  in  which  the  main  stem  itself  creeps 
on  or  just  under  the  ground,  striking  root  and  sending  up  shoots  as 
it  goes,  thus  spreading  its  own  growths  to  new  ground.  This  is 
the  way  in  the  Grasses,  whose  creeping  stems  run  and  branch  so 
freely,  and  interlock  so  closely,  that  they  form  the  dense  mats  we 


382  The  Living  Plant 

call  turf.  Our  native  ferns,  as  well,  have  stems  that  creep,  and 
send  up  the  beautiful  fronds  from  new  soil.  There  are  other 
plants,  like  Solomon's  Seal,  which  grow  onward  under  ground 
year  after  year,  the  old  parts  dying  behind  as  the  new  are  devel- 
oped in  front;  and  such  plants  may  wander  a  considerable  dis- 
tance through  the  woods,  carrying  their  new  branches  ever  into 
new  ground.  In  the  tropical  forests  there  are  epiphytes  which 
wander  in  this  manner  over  tree  trunks,  and  certain  undergrowth 
kinds  which  grow  forward  a  little  on  stilt-like  aerial  roots. 


Fio.  148. — Fairy  Rings,  of  Mushrooms,  originating  as  explained  in  the  text.  Three  com- 
plete rings  and  a  partial  one  appear  in  the  picture.  (Reduced  from  Kerner's  Pflan- 
zenleben.) 

Roots  possess  a  certain  power  of  shortening  their  length  during 
later  growth,  and  advantage  thereof  is  taken  by  some  bulbous 
plants,  like  the  Tulips.  New  bulblets  are  formed  in  the  axils  of 
the  scales  of  the  old  ones,  and  then  are  pulled  an  appreciable  dis- 
tance from  the  parent  by  the  shortening  of  their  own  radiating 
roots  after  these  have  become  fixed  in  the  soil.  And  there  are 
other  minor  ways  in  which  growth  helps  to  spread  plants,  though 
I  think  the  aforementioned  include  all  of  real  consequence. 


How  Plants  Secure  Change  of  Location          383 

It  is  obvious  that  the  ways  described  in  this  section,  while 
efficient  so  far  as  they  go,  secure  no  wide  spread  for  plants;  they 
are,  indeed,  supplementary,  or  extra,  methods  which  happen  to 
be  rendered  available  by  some  peculiarity  of  growth  or  habit  in 
the  plants  concerned,  all  of  which 
possess  also  the  far  more  efficient 
methods  we  are  now  to  consider. 
As  to  these  latter,  they  depend 
in  reality  upon  a  single  principle. 
Forbidden  by  their  mode  of  life 
to  move  when  adult,  plants  have 
taken  advantage  of  that  stage  in 
their  lives  when  they  are  small 
and  therefore  easily  transportable, 
— the  stage  of  the  embryo.  This 
embryo,  with  its  vitality  sus- 
pended for  a  time,  together  with 
its  store  of  food  substance  and 

protective    COatS,    Constitutes    the    FIG.    149.— Pod  of  a  Vetch,  explosively 
0        ,          i  •    i      •  j    f  propelling  its  seeds. 

Seed,  which  is  severed  from  the 

plant  and  can  then  be  transported  in  various  ways  as  we  shall 

now  proceed  to  consider. 

3.  Projection  by  Elastic  Machinery. — The  appreciable  size  and 
weight  of  seeds  makes  possible  their  projection  to  some  dis- 
tance by  a  sudden  application  of  sufficient  power;  and  this  fact 
has  been  readily  turned  to  use  by  plants  for  their  dissemination. 
The  propelling  machinery  is  variously  made.  In  some  kinds  of 
seed-pods,  definite  bands  of  cells  ripen  in  a  state  of  stretched 
tension,  which  presently  becomes  so  great  that  the  pod  bursts 
suddenly,  hurling  out  the  loose-lying  seeds  to  a  distance  of  several 
feet.  In  the  Vetches,  the  two  halves  of  the  pea-like  pods  twist 
suddenly  apart  in  opposite  directions  (figure  149);  in  the  Wild 
Geranium,  the  ripening  styles  suddenly  curl  up  from  the  length- 
ened receptacle  (figure  150);  the  valves  of  the  capsules  burst 


384  The  Living  Plant 

apart  explosively,  shooting  out  the  seeds,  in  the  Castor  Bean, 
the  Witch  Hazel,  the  Acanthus  (figure  151),  or  the  West  In- 
dian "Sand  Box,"  whose  report  is  said  to  rival  that  of  a  pistol. 
In  all  of  these  cases,  the  propulsion  of  the  seeds  may  be  seen, 

heard,  and  even  smartly  felt  by  the 
reader,  if,  when  the  pods  are  near 
ripeness,  he  will  bring  them  to  the 
room  where  he  spends  most  of  his 
time.  In  the  Violets  the  sides  of 
the  ripening  pods  come  to  press 
harder  and  harder  upon  the  smooth 
seeds  which  are  held  in  the  angle 
between  them,  until  finally  the  seeds 
are  shot  out  of  the  pods  in  precisely 
the  same  way  that  a  smooth  bean 

FIG.  150.— The  seed-propelling  fruit  of  .          ,      ,    .  ,      . 

wild  Geranium,  explained  in  the  or  a  nut  can  be  shot  from  between 
the  tightly-pressed  fingers  (figure 

152).  In  all  of  these  cases,  the  seeds  show  approach  to  the 
qualities  best  in  all  shot, — they  are  round,  smooth  and  relatively 
heavy. 

Instead  of  the  springing  force  of  elastic  dry  tissues,  some  plants 
make  use  of  turgescence, — that  is,  of  the  pressure  developed  by 
tensely-gorged  cells  against  lines  of  a  weaker  sort,  ending  in  ex- 
plosive rupture  and  flight  of  the  seeds.  This  is  very  well  known 
in  the  fruits  of  the  Jewel-weed,  called  also  Touch-me-not,  a 
common  wild  plant  which  takes  its  name  from  the  habit.  In  the 
descriptively-named  "Squirting  Cucumber"  of  the  East,  the 
entire  pulpy  contents  inside  of  the  firm-skinned  fruit  ripen  so 
turgidly  that  pulp  and  seeds  together  squirt  out  to  a  distance 
when  an  outlet  is  made  by  the  breaking  of  the  fruit  away  from  its 
stem  (figure  153). 

Related  to  these  ways  in  its  principle,  though  differing  much 
in  detail,  is  the  method  used  in  those  cases  where  small  round 
and  relatively  heavy  seeds  come  to  lie  loosely  in  open-topped 


How  Plants  Secure  Change  of  Location  385 

capsules  on  long  stiff-elastic  stalks.  When  the  stalks  are  shaken 
by  gusts  of  wind,  or  the  impact  of  animals  passing,  the  seeds  are 
thrown  out  by  the  movement,  especially  the  jerky  recoil.  The 
exit  of  the  seeds  from  some  of  these  pods  lies  along  smooth 
grooves  so  placed  as  to 
guide  the  seeds  at  an 
angle  best  for  their  flight 
to  a  distance.  These  fea- 
tures appear  well  in  the 
Poppies  (figure  154), 
upon  which  observation 
and  experiment  are  easy ; 
but  it  really  is  one  of  the 
commonest  of  the  modes 
of  dissemination,  prevail- 
ing through  several  large 
families  of  plants,  nota- 
bly the  Figworts,  Bell- 
worts,  Primroses  and 

Pinks.      And  Other  meth-    FIG.    151.— The  pods  of  Acanthus  explosively  pro- 

ods  of  projection  occur, 

as  the  reader  may  see  for  himself  in  the  field,  or  find  described  in 

the  works  on  the  subject. 

A  special  form  of  projection  through  movements  of  ripening 
tissues  is  shown  by  those  seeds  which  are  pushed  along  the  ground 
by  movements  of  hygroscopic  hairs.  The  causes  of  hygroscopic 
movements  were  considered  in  the  chapter  on  Absorption;  and 
it  will  here  suffice  to  say  that  some  tissues,  by  absorbing  moisture 
from  the  air,  or  giving  it  up  thereto,  can  swell  and  twist  very 
forcibly,  though  not  suddenly.  In  some  Clovers  hygroscopic 
hairs  are  so  placed  in  conjunction  with  backwardly-directed 
parts,  which  act  as  "chocks,"  that  every  movement  of  the  hairs 
pushes  the  seed  along  the  ground  (figure  155).  The  arrangement 
is  yet  better  in  the  curious  "living  Oat"  (Avena  sterilis},  which  can 


386 


The  Living  Plant 


creep  appreciable  distances  within  a  few  hours,  and  which,  when 
placed  with  clothes  in  a  drawer,  burrows  among  these  in  a  fashion 
quite  uncanny.  Hygroscopic  movements  also  aid  dissemination 
indirectly;  for  hygroscopic  hairs,  or  equivalent  structures,  in  some 


FIG.  152.— The  pod  of  a  Violet  projecting 
its  seeds  by  a  method  explained  in  the 
text. 


FIG.  153. — The  Squirting  Cucumber,  pro- 
jecting its  seeds  as  explained  in  the 
text. 


Orchids,  Mosses  and  other  plants,  push  seeds  or  spores  from  the 
interior  of  the  capsules  to  the  surface,  where  they  can  be  reached 
and  transported  by  action  of  the  wind.  Hygroscopic  movements 
have  also  a  part  in  the  final  bursting  of  seed  pods  in  some  of  the 
cases  described  a  page  or  two  earlier. 

But  the  method  of  projection,  though  effective  in  principle, 
has  marked  limitations,  since  the  maximum  distance  to  which 
seeds  can  be  thrown,  no  matter  how  great  the  power  may  be,  does 
not  exceed  a  dozen  or  two  feet.  This  is  enough  for  very  small 
plants  and  limited  spread,  but  does  not  suffice  for  much  larger 
kinds  or  a  wider  dispersal.  It  is  fortunate,  therefore,  that  a 


How  Plants  Secure  Change  of  Location  387 

method  more  effective  is  available.  This  method,  in  brief,  is 
this, — the  provision  of  appliances  which  ensure  that  the  seeds 
shall  be  carried  away  by  moving  agencies  that  exist  in  the  world 
around.  These  agencies  are  principally  three,— winds,  water- 
currents  and  animals.  There  is  also  a  fourth,— gravitation,— 
but  it  acts  in  a  profitless  direction,  although  indirectly  it  con- 
tributes some  aid  to  dissemination  by  making  round  seeds  roll 
down  slopes,  and  causing  elastically-walled  heavy  seeds,  such  as 


FIG.   154. — A  pod  of  a  poppy,  showing 
openings  for  exit  of  the  seeds. 


FIG.  155. — A  fruit  of  Clover,  showing  the 
hygroscopic  hairs. 


many  trees  have,  to  rebound  from  hard  surfaces  with  a  force 
which  must  oftentimes  remove  them  considerably  away  from  the 
plant  that  produced  them.  The  other  three  agencies,  however, 
are  vastly  important  in  dissemination,  as  the  following  descrip- 
tions will  amply  attest. 

4.  Waftage  by  Winds. — Of  the  motive  forces  of  nature,  winds 
are  one  of  the  most  ubiquitous,  and  the  easiest  of  all  for  plants 
to  make  use  of.  They  occur  in  all  grades  from  the  wildest  of  gales, 
creating  disturbance  through  hundreds  of  miles,  down  to  the 
faintest  of  zephyrs  confined  to  a  limited  region;  and  they  include 
as  well  those  upward  currents  of  air  which  rise  over  heated  places 
in  summer  to  a  height  where  wide-ranging  breezes  prevail.  To 
utilize  these  winds  for  their  spread,  plants  have  only  to  attach 


388  The  Living  Plant 

to  their  seeds  such  devices  as  shall  make  them  expose  a  great 
surface  in  proportion  to  weight,  and  this  they  have  done  in  mani- 
fold ways. 

The  simplest  way  of  causing  a  seed  to  expose  much  surface  to 
wind  lies  hi  the  addition  of  a  broad  flat  sail,  or  a  wing.  Everybody 
knows  the  seed  of  the  Maple,  with  the  lengthened  wing  growing 
out  from  the  wall  of  the  fruit  (figure  156),  and  the  Elm,  with  a 
similar  wing  except  that  it  encircles  the  fruit.  The  conspicuous 


Fio.  156.— Winged  fruit 

of  a  Maple.  FIG.  157. — Winged  fruits  of  the  Linden. 

way  in  which  these  seeds  in  their  season  are  blown  about  our 
streets  proves  the  efficiency  of  the  arrangement.  The  seeds  of 
the  Linden,  or  Basswood,  are  likewise  transported  by  a  very  fine 
wing,  made  from  a  bract  grown  fast  to  the  stalk  (figure  157), 
while  in  Pine  and  Catalpa  the  wings  grow  out  from  the  coats  of 
the  seed.  These  are  representative  examples;  and  there  are 
others  as  well,  but  less  common,  in  which  the  wing  is  supplied 
by  calyx  or  corolla. 

Acting  like  the  wings,  and  in  some  ways  still  more  simple  and 
effective,  are  large  bladders,  in  which  the  seeds  lie.  Some  ap- 
proach thereto  is  made  by  those  kinds  of  the  Pea  Family  which 
have  pods  greatly  swollen  but  very  small  seeds;  but  it  reaches 
more  typical  development  in  cases  like  the  Bladder  Nut,  where 


How  Plants  Secure  Change  of  Location  389 

the  ovary  forms  a  very  loose  envelope,  or  in  some  Orchids,  where 
it  is  made  from  a  greatly  inflated  seed  coat  (figure  158).  In 
all  of  these  cases  the  principle  is  the  same, — that  of  a  great  spread 
of  surface  accompanied  by  very  light  weight. 

Another  fine  method  for  giving  much  surface,  consists  in  the 
provision  of  long  soft  hairs,  or  plumes;  and  seeds  displaying  this 
arrangement  are  plenty.  The  Cotton  seed,  for  example  (fig- 
ure 159),  develops  hairs  of  such  number  and  length  that  they 
serve  not  only  to  spread  it  afar  under  action  of  wind,  but  prove 


FIG.  158.— The 
seed  of  an  Or- 
chid, showing 
through  its 
loose  b  1  a  d- 
der-like  coat. 


FIG.   159. — A   cotton   seed,  with 
its  long  soft  hairs. 


FIG.  160.  —  A 
plumose  fruit 
of  Clematis. 


FIG.  161.— The 
parachute 
fruit  of  the 
Dandelion 


incidentally  a  great  utility  to  man,  since  they  yield  him  the  fiber 
for  the  commonest  of  all  of  his  fabrics.  The  familiar  silky  plume 
of  the  Clematis  (or  Virgin's  Bower)  is  made  by  the  outgrowth  of 
hairs  from  the  style  (figure  160);  the  parachute  plume  of  the 
Dandelion  from  the  calyx;  the  soft  tuft  of  the  Milkweed  from  the 
seed  coat;  the  nebulous  mass  of  the  Smoke  bush  from  stalks  of 
unfruitful  flowers.  The  phrase  "parachute  plume"  used  above 
was  carefully  chosen  because  of  its  suitability.  For  in  the  Dan- 
delion (figure  161),  and  some  other  plants,  the  plumes  are  spread 
out  horizontally,  and  keep  that  position  in  flight  because  of  the 


390  The  Living  Plant 

weight  of  the  seed  which  hangs  some  distance  below.  Thus  the 
seeds  can  be  lifted  over  the  tops  of  the  trees  by  those  currents  of 
air  which  rise  upward  from  places  that  are  heated  in  summer, 
whence  the  wind  may  transport  them  to  far  distant  parts.  It  is 
largely,  no  doubt,  because  the  Dandelion,  and  its  relatives  in  the 
family  Compositae,  have  developed  so  efficient  a  mode  of  wind 
transport,  that  they  constitute  the  largest  and  most  widely 
diffused  of  alt  the  existent  plant  families.  And  it  is  also  of  interest 
to  note  that  these  wind-carried  seeds  exhibit  a  very  effective 
secondary  adaptation  to  wind-transport,  namely,  great  lightness 
of  build,  which  even  extends  to  the  employment  of  oil,  a  lighter 
material  than  starch,  as  food  in  reserve  for  the  embryo.  And 
other  modifications  of  this  principle  of  plume-transport  exist, 
as  the  reader  may  learn  from  his  own  observation,  or  the  several 
good  books  on  the  subject. 

If  one  compares  the  habits  of  plants  whose  seeds  are  winged  or 
are  plumed,  respectively,  he  will  find  that  in  general  the  winged 
seeds  belong  to  trees  and  the  plumed  seeds  to  herbs.  It  is  easy  to 
imagine  a  reason  for  this.  Plumes  are  a  better  lifting  device  than 
are  wings.  The  extra  height  and  better  exposure  to  wind  of  the 
tree  gives  its  winged  seeds  a  start  which  is  ample  for  transport  to 
sufficient,  if  not  to  the  greatest  distances.  Moreover,  some  tree 
seeds  possess  such  a  relation  of  weight  to  wing  form  that  they  do 
not  fall  directly  to  earth,  but  only  after  whirling  through  long 
spirals,  thus  giving  the  wind  a  longer  action  upon  them.  But 
with  herbs  in  their  low  sheltered  positions,  the  wing  would  be  far 
less  efficient;  and  the  lifting  action  of  plumes  is  a  notable  ad- 
vantage. The  plume  is  actually  a  better  device  than  the  wing, 
and  there  is  reason  to  think  it  a  later  evolutionary  development ; 
for  our  modern  herbs,  I  believe,  appeared  later  in  time  than  our 
trees. 

There  is  still  another  method,  quite  different  in  principle,  of 
increasing  the  surface  in  proportion  to  weight.  It  consists  in 
excessive  reduction  in  size.  It  is  a  mathematical  fact  that  as  a 


How  Plants  Secure  Change  of  Location  391 

sphere  or  other  rounded  body  diminishes  in  size,  its  bulk,  and 
therefore  its  weight,  diminishes  far  faster  than  its  surface;  or,  in 
other  words,  the  smaller  such  a  body  becomes  the  more  surface 
does  it  spread  in  relation  to  weight.  A  body  has  only  to  reach  a 
certain  point  of  smallness,  therefore,  when  the  very  slightest  air 
movements  are  enough  to  blow  it  away,  and  to  keep  it  suspended 
indefinitely  in  the  air.  This  is  the  reason  that  dust  floats  as  it 
does;  and  amongst  this  dust,  and  a  part  of  it,  float  the  spores  of 
Bacteria,  Molds,  Yeasts,  Ferns  and  other  spore-bearing  plants, 
which  depend  on  this  method  for  their  dissemination.  There  can 
be  little  wonder  that  such  plants  are  found  so  widely  distributed 
when  we  remember  how  far  this  dust  can  be  carried  by  any  sum- 
mer breeze.  Among  seed-bearing  plants,  however,  the  habit  of 
forming  a  many-celled  embryo  before  separation  of  the  seed 
from  the  parent  plant,  makes  the  seed  too  large  for  this  method 
to  be  used,  though  in  some  Orchids  the  embryo  formation  is  post- 
poned, leaving  the  seed  small  enough,  especially  when  a  loose 
open  sac  is  added,  to  be  transported  in  this  manner. 

The  reader  who  is  versed  in  morphology  will  observe  that  I 
often  ignore  the  distinction  between  the  seed  and  its  accom- 
panying fruit.  From  the  point  of  view  of  the  principle  and  effi- 
ciency of  dissemination,  it  makes  no  particular  difference  whether 
the  disseminating  mechanism  is  formed  from  a  part  of  the  seed 
itself,  or  from  the  associated  receptacle,  ovary,  style,  calyx,  or 
corolla.  And  if  one  asks  why  a  particular  plant  forms  its  wing  or 
its  plume  in  this  way,  and  another  in  that,  we  can  only  reply  that 
herein  lies  another  illustration  of  the  first  law  of  adaptation, — 
that  a  new  structure  when  needed  is  formed  from  the  part  which 
happens  to  be  most  available  for  the  purpose,  and  sometimes  that 
part  is  one  thing  and  sometimes  it  is  another.  Next  after  the 
seed  itself,  however,  the  disseminating  mechanism  is  most  often 
constructed  from  the  part  next  contiguous,  the  ovary;  and  the 
frequency  of  its  use  for  this  purpose  has  caused  its  retention  with 
the  seeds  long  after  the  other  parts  have  fallen.  It  is  this  per- 


392  The  Living  Plant 

sistent  ovary,  modified  to  aid  dissemination,  and  often  accom- 
panied by  contiguous  parts  of  the  flower,  which  constitutes  the 
fruit  of  the  plant. 

For  the  sake  of  completeness  I  should  add  yet  another  to  the 
methods  by  which  the  wind  aids  dissemination,  although  it  is 

only  of  minor  importance.  Not 
only  seeds,  but  some  other  parts 
of  plants  which  are  capable  of 
growth,  are  also  transported  by 
winds,  especially  when  these  rise 
into  gales.  It  is  thus  with  some 
leaves,  in  Begonias  and  Life-plants 
(Bryophyllum) ;  joints  of  stem  in 
some  Cactuses;  buds  or  bulblets  in 

Fio.    162. — The   Rose   of   Jericho,    ex- 
plained in  the  text.     (Reduced  from  some  Sedums  and  Lilies;  the  brittle 

Kerner's  Pflanzenleberi) .  .  ~  -^7.,,  mi       r      ,    , ,      ,   • 

twigs  of  Willows.     The  fact  that  in 

such  cases  the  transport  is  incidental  rather  than  adaptively  de- 
veloped makes  it  none  the  less  real;  while  moreover,  the  very  acci- 
dentality  of  the  method  illustrates  to  perfection  the  way  in  which 
many,  and  perhaps  most,  adaptations  begin.  Again,  some  kinds 
of  plants  that  live  in  open  dry  places  roll  their  branches  inwards  at 
times  to  form  a  kind  of  ball,  and  in  this  state  may  be  blown  from 
their  anchorage  and  sent  rolling  across  plains  or  the  frozen  snow, 
to  take  root  again  in  new  places,  often  scattering  their  seeds  or 
spores  as  they  go.  Such  plants,  called  "  tumble-weeds "  and  es- 
pecially characteristic  of  prairies  or  plains,  are  well  exemplified  in 
the  Russian  Thistle,  a  troublesome  new  weed  of  the  west,  the  Rose 
of  Jericho,  mentioned  at  times  in  the  Scriptures  (figure  162),  and 
the  Resurrection  Plant  of  the  southwest.  Sometimes  it  is  not  the 
whole  plant  but  only  its  fruit-cluster,  as  in  members  of  the  Parsley 
Family,  which  is  thus  broken  loose  and  sent  rolling  away.  A 
simpler  method  is  displayed  in  those  flat  pods,  such  as  some 
Locusts  possess,  which,  curling  into  loose  spirals  that  catch  every 
wind,  are  rolled  over  smooth  ground  or  the  snow. 


How  Plants  Secure  Change  of  Location  393 

5.  Flotage  upon  Water. — When  water  moves  onward  in  currents, 
whether  merely  those  minor  and  temporary  kinds  made  by  winds 
in  their  sweep  over  lakes,  and  by  rains  in  the  rivulets  they  cause, 
or  in  the  mighty  and  permanent  streams  of  river  and  ocean,  it 
forms  a  good  agency  of  transport  even  though  less  widely  useful 
than  winds. 

With  water-currents,  as  with  others  of  the  methods,  some 
transport  is  incidental,  as  when  floods  tear  out  whole  plants  from 
the  banks  and  leave  them  to  grow  anew  in  another  location  when 
the  waters  subside,  or  as  happens  when  rivers  sweep  broken 
twigs  of  Willow  to  new  places  where  they  readily  strike  root. 
Again,  rain  drops  splash  out  from  open  capsules,  spores  or  small 
seeds  which  are  carried  away  in  rivulets  to  places  where  they  are 
left  in  dampness  good  for  their  germination.  Thus  also  are 
carried  little  buds  (gemmae)  of  the  Liverworts,  and  probably  the 
axillary  bulblets  produced  by  several  kinds  of  plants.  Besides, 
most  wind-scattered  seeds  are  so  light  that  they  float  well  upon 
water,  and  thus  effect  a  still  wider  transportation.  But  these  in- 
cidental methods  are  insignificant  in  comparison  with  those  which 
are  secured  by  adaptation  in  the  plants. 

In  the  first  place,  some  kinds  secure  transport  by  water-currents 
through  their  very  habit  of  life,  which  indeed  may  be  partly  de- 
termined to  this  end.     Such  are  the 
free-living  submerged  Algae,  which  in- 
clude vast  numbers  of  the   simpler 
kinds  of  Seaweeds,  and  the  free-float- 
ing plants  like  our  Duckweeds  (figure 
163)  and  the  Water  Hyacinth,  with  FIG  163  _The  Duckweed,  a  float- 
similar  kinds  of  the  tropics.     And  a     ing  Plant-     (Copied   from   the 

.       .  Chicago  Textbook.) 

modification  of  this  habit  is  found 

in  some  sorts  of  our  own,  like  some  Watercresses,  which  are  only 
lightly  attached  and  are  readily  moved  to  new  places;  while  many 
kinds  of  our  water-weeds  form  naturally-detachable  buds  which 
are  easily  floated  afar. 


394 


The  Living  Plant 


But  the  most  perfect  transportation  by  water  is  found  in  those 
cases  where  seeds  are  adapted  expressly  to  this  method,  which 
requires  some  kind  of  a  float,  and  a  power  to  resist  decay  for  a 
considerable  time.  Thus  in  the  African  Lotus,  or  Nelumbium 
(figure  164),  the  great  top-shaped  and  air-filled  receptacle,  well 
known  from  its  conventionalized  use  in  the  art  of  the  East,  forms 
a  very  effective  float  for  the  seeds  which  are  dropped  here  and 


FIG.  164.— The  floating  receptacle  of  Nelumbium, 
showing  a  part  of  the  seeds. 


FIG.  165.— The  seed  of 
a  water-lily,  with  its 
flotation  bladder. 
(Copied  from  Gray) . 


there  as  it  goes.  In  some  Water-lilies,  the  float  is  an  air-filled 
bladder  formed  by  a  loose  seed  coat  around  each  single  seed 
(figure  165) ;  and  in  other  commoner  kinds  the  float  is  a  swollen 
wall  of  the  ovary.  In  the  Cocoanut  the  great  air-filled  husk  is  a 
development  of  the  ovary,  and  so  perfect  a  flotation  device  that 
this  plant  forms  the  chief  one  of  the  palms  that  rise  in  the  tropic 
isles  throughout  the  seven  seas.  So  perfect,  by  the  way,  is  the 
power  of  this  husk  to  resist  decay  in  the  water,  that  a  cordage, 
called  coir,  is  made  therefrom  for  special  use  where  resistance  to 
decay  in  salt  water  is  particularly  needed.  And  by  analogous 
methods  other  seeds  as  well  have  been  carried  over  vast  reaches  of 
ocean. 

6.  Carriage  by  Animals. — Most  ubiquitous  of  all  of  the  moving 


How  Plants  Secure  Change  of  Location          395 

agencies  of  nature,  so  far  as  utilization  by  plants  is  concerned, 
are  animals,  which  forever  are  roaming  among  plants  in  their 
search  for  food  or  for  shelter.  And  in  general  where  plants  are 
most  plenty,  there  animals  too  most  abound.  There  is,  by  the 
way,  a  kind  of  poetical  justice  in  plants 
making  animals  do  service  for  them  as 
some  return  for  the  priceless  benefits  they 
confer  upon  animals. 

The  ways  in  which  animals  are  made 
to  cooperate  in  the  dissemination  of 
plants  are  various.  In  the  first  place,  as 
in  case  of  other  methods,  some  transport 
is  incidental,  that  is,  it  occurs  without  FlG  1G6._ The  hooked  fruits 
the  existence  of  any  particular  adaptations  of  Burdock- 

thereto.  Thus  the  seeds  of  some  water  plants  are  carried  vast  dis- 
tances embedded  in  the  mud  which  adheres  to  the  feet  of  the  larger 
and  wide-ranging  water  birds,  some  of  which  have  been  shot  with 
such  seeds  attached  to  their  feet  or  their  feathers.  Again,  some 
heavier  seeds,  such  as  nuts,  are  carried  away  by  squirrels  or  birds 
to  be  eaten  elsewhere  or  stored  up  for  winter;  but  some  are  dropped 
on  the  way  and  others  never  are  used,  so  that  they  come  to  grow 
in  new  places.  Probably  also  the  scattering  of  spores  of  Mildew, 
or  other  leaf  Fungi,  by  temporary  adhesion  to  the  slimy  bodies  of 
snails  is  of  similar  nature. 

Turning  now  to  the  definite  adaptations  which  fit  seeds  for 
transport  by  animals,  we  find  first  of  all  a  simple  and  obvious 
method  in  the  provision  of  hooks  or  other  arrangements  suitable 
for  attachment  of  seeds  to  fur  or  to  feathers.  Everybody  will 
recall  the  case  of  the  close-clinging  Burdock  (figure  166),  while  the 
Cocklebur  and  the  Agrimony  are  equally  efficient.  Some  striking 
examples  occur  in  the  plants  of  great  plains,  where  large  animals 
are  especially  abundant,  as  for  instance  the  Unicorn  Plant  of  the 
west  (figure  167)  which  catches  in  the  tails  of  horses,  and  the 
Grappling  plant  of  South  Africa  (figure  168),  which  entangles 


396 


The  Living  Plant 


itself  in  the  fur  of  lions.  But  innumerable  small  plants  use  this 
method,  as  one's  clothing  bears  visible  testimony  after  rambles 
through  fields  in  the  autumn.  The  hooks  are  most  diverse  in 
form  as  well  as  morphological  origin,  some  coming  from  seed 
coat,  some  from  ovary,  some  from  calyx,  some  from  bracts, — no 
doubt  in  each  case  along  the  lines  of  development  that  were 
easiest  at  the  moment.  However  tightly  these  hooks  may  cling, 


FIG.   167.— The  Unicorn    plant,  ex- 
plained in  the  text. 


FIG.  168. — The  Grappling  plant,  ex- 
plained in  the  text.  (Copied  from 
Miss  Stoneman's  Plants  of  South 
Africa.) 


the  seeds  sooner  or  later  fall  to  the  ground,  either  brushed  away 
by  contact  with  some  hard  object,  or  else  dropped  when  the  hair 
of  the  animal  is  shed.  Nor  is  the  employment  of  hooks  confined 
only  to  seeds,  for  they  exist  also  on  some  separable  joints  of  small 
Cactus,  or  the  slender  stems  of  the  Bedstraw  or  "Tear-thumb," 
both  of  which  secure  some  transport  through  the  contact  of 
wandering  animals. 

Hooks  are  efficient  with  fur  but  less  so  with  feathers,  to  which 
some  adhesive  material  is  better  adapted.  Thus,  seeds  which  are 
carried  by  birds  commonly  possess  a  covering  of  mucilage,  as  hi 
very  many  water  plants;  and  so  effective  is  this  method,  in  con- 
junction with  that  where  the  adhesive  is  simply  the  mud  of  a 
pond,  that  water  plants  are  among  the  most  widely-distributed 


How  Plants  Secure  Change  of  Location  397 

of  all  living  things,  some  kinds  actually  occurring  in  all  of  the 
continents.  Especially  effective  is  the  very  sticky  " bird-lime," 
formed  by  Mistletoe  berries  and  many  other  parasites;  such  seeds 
adhere  to  the  feet  or  feathers  of  birds  and  thus  obtain  attachment 
upon  trees,  the  only  positions  in  which 
they  can  grow.  Some  low-growing 
herbs  like  the  Twin-flower  (figure  169), 
attain  the  same  end  by  the  possession  of 
adhesive  glands  on  the  fruit. 

There  remains  but  one  other  method  of 
utilizing  animals  in  the  transport  of  seeds, 
and  that  is  the  most  striking  and  im- 
portant of  all.  It  consists  in  providing 
the  seed  with  some  form  of  indigestible 
covering,  surrounding  the  same  with  a 
nourishing  and  appetizing  pulp,  and  giv- 
ing the  whole  a  bright  color  which  con- 
spicuously displays  its  position.  Such 
fruits  are  then  eaten  by  animals,  and  the 
seeds  pass  through  their  bodies  unin-  FlG  169._The  gianduiar-ad- 
jured,  after  an  interval  that  usually  en-  *fsive  fruits  of  the  Twin- 

7  _  t  flower. 

sures  their  discharge  at  a  place  con- 
siderably distant  from  where  they  were  eaten.  This  without 
doubt  is  the  explanation  of  the  existence  and  characteristics  of 
colored  and  edible  fruits  in  nature;  and  so  abundant  and  familiar 
are  they  that  we  need  hardly  cite  any  examples.  So  common, 
indeed,  are  edible  fruits,  and  so  effective  their  use,  that  this 
method  of  dissemination  must  rank  very  high  among  the  modes 
of  plant  transport,  and  is  second,  if  to  any,  only  to  wind  waftage. 
To  this  method  of  seed  transport,  birds  are  better  adapted  than 
other  animals,  since  their  smaller  size  makes  it  possible  to  attract 
them  with  not  too  lavish  provision  of  pulp,  and  their  very  active 
habits  ensures  their  movement  over  considerable  spaces.  Ac- 
cordingly, colored  fruits  are  especially  abundant  on  trees,  shrubs, 


398  The  Living  Plant 

and  tall-growing  vines,  where  birds  most  frequent;  though  they 
are  by  no  means  absent  from  low-growing  herbs  where  they  are 
eaten  by  ground  birds  or  some  of  the  smaller  mammals.  The 
indigestible  coatings  are  formed  either  by  seed  coats,  as  in  Grape, 
by  ovary  walls,  as  in  Strawberry,  or  by  a  part  of  the  ovary,  as  in 
the  stone  of  the  Cherry.  Sometimes  instead  of  the  hard  coat,  an 
inedible  core  is  developed,  which  is  carried  away  but  not  eaten, 
as  in  Apples,  while  in  yet  others  the  slippery  seeds  are  hardly 
swallowed  at  all,  but  are  scattered  around  as  the  pulp  is  devoured, 
as  in  Oranges.  The  pulp  is  formed  from  the  most  diverse  parts, — 
one  can  almost  say  every  possible  part, — from  ovary  as  in  Grape, 
receptacle  as  in  Strawberry,  bract  as  in  Juniper,  seed  coat  as  in 
Yew,  calyx  as  in  Wintergreen,  placentae  as  in  Watermelon,  or 
hairs  as  in  Oranges.  The  colors  in  general  are  such  as  are  most 
conspicuous  under  the  special  conditions  prevailing  where  the 
fruit  ripens.  Thus  red  is  the  most  common  of  the  colors  of  fruits, 
and  it  is  that  which  is  most  conspicuous  against  the  green  of 
foliage;  but  purple  or  blue  is  more  common  in  fruits  of  the  autumn 
which  ripen  when  the  foliage  has  turned  yellow  or  red,  while 
white  occurs  in  some  berries  which  grow  in  the  dusk  of  shady 
places  near  the  ground.  Before  they  are  ripe  these  fruits  are 
commonly  sour,  or  astringent  and  unpalatable,  and,  moreover, 
are  green  in  color,  precisely  like  the  foliage.  This  color  may  serve 
to  prevent  their  notice  by  animals  before  the  seeds  are  ripe,  al- 
though such  a  function  for  the  green  color  is  probably  wholly 
incidental  and  secondary  to  its  use  as  accessory  food-making 
tissue. 

A  special  phase  of  dissemination  by  animals,  the  importance  of 
which  has  only  lately  been  realized,  is  the  transport  of  seeds  of 
low-growing  herbs  by  ants.  Such  seeds  are  mostly  small  and 
light,  but  are  provided  with  an  attached  reservoir  of  food-material 
(called  the  caruncle),  attractive  to  ants,  which  carry  the  seeds  to 
various  distances  from  the  capsules,  leaving  them  where  the  food 
has  been  used.  It  has  also  been  supposed  that  some  seeds  which 


How  Plants  Secure  Change  of  Location  399 

happen  to  resemble  the  larvae  of  ants  are  transported  some  dis- 
tance towards  their  nests  by  the  same  more  industrious  than 
sensible  insects.  It  is  possible,  furthermore,  that  a  somewhat 
similar  explanation  applies  to  certain  seeds  or  fruits  which  look 
remarkably  like  beetles,  as  do  some 
Castor  Beans,  or  like  caterpillars, 
as  do  some  members  of  the  Pea 
Family  (figure  170) ;  for  such  seeds, 
which  are  protected  by  hard  coats 
against  digestion,  are  supposed  by 
some  naturalists  to  be  swallowed 
by  birds  in  the  belief  that  they  are  FlG-  170;— The  pod  of  Scorpiwrus,  SUP- 

J  ,         posed  to  resemble  a  caterpillar. 

really  live  insects.  Again,  brightly- 
colored  hard  seeds,  protectively  coated,  appear  to  be  swallowed  by 
birds,  as  are  other  bright  objects,  simply  because  of  their  attract- 
ive or  conspicuous  appearance.  But  these  latter  matters  are  doubt- 
ful, and  perhaps  are  fancies  rather  than  facts,  though  we  must  re- 
member that  strangeness  or  seeming  improbability  are  not  valid 
scientific  objections  to  any  explanation  of  a  natural  phenomenon. 
Not  only  birds  and  small  mammals,  but  also  bats,  snails,  insects, 
fish,  and  perhaps  other  animals,  have  been  detected  in  carrying 
seeds  by  some  one  or  the  other  of  the  various  ways  we  have 
mentioned.  The  subject  is  by  no  means  exhausted,  and  most 
interesting  discoveries  without  doubt  still  await  the  keen-sighted 
and  persistent  observer. 

To  complete  the  subject  of  transport  by  animals  we  must 
mention  the  action  of  man,  though  his  agency  is  of  course  in- 
cidental and  not  adaptational.  Unintentionally  he  has  spread 
weeds  from  country  to  country,  until  some  occur  all  around  the 
world,  while  deliberately  he  has  carried  the  plants  that  are  valua- 
able  to  him  to  all  parts  of  the  earth.  Indeed,  upon  this  latter  end 
he  concentrates  much  effort  and  thought,  reaching  their  culmi- 
nation in  the  deliberate  and  systematic  attempts  of  our  national 
Department  of  Agriculture  to  gather  useful  plants  from  all  parts 


4oo  The  Living  Plant 

of  the  world,  and  to  establish  in  this  country  every  kind  of  plant 

which  can  possibly  be  of  service  to  our  people. 

Before  leaving  dissemination  it  is  desirable  to  note  certain 

adaptations  which  are  correlated  therewith,  though  hardly  a 
part  of  transport  itself.  Thus,  some  seeds 
seem  to  possess  a  certain  power  of  plant- 
ing themselves  through  the  movements  of 
a  definite  hygroscopic  mechanism  which  is 
so  built  as  to  bore  the  seeds  into  the  ground  ; 
the  wild  Erodium,  and  the  grass  Stipa 
pinnata  (figure  171)  are  examples.  In  other 
cases  the  ripening  fruit-stalk  turns  away 
from  the  light,  and  thus  carries  the  seeds 
into  clefts  of  the  rocks  or  the  cliffs  on 
which  the  plants  grow,  thus  ensuring  their 
fall  in  a  place  conformable  to  the  habits 

FIG.  171.—  The  fruit  of  Stipa       P    Jt  ,  ..  .  ,.    ,.         T. 

pinnata,  supposedly  self-  of  the  plant;  this  is  true  of  the  Linana 


Cymbalaria  of  Europe,  already  described 
in  another  connection  (figure  81).     Some 

plants  place  the  seed  pods  in  a  protective  position  while  ripening. 
This  is  common  in  water  plants,  which  draw  the  fruit  under 
water  by  a  spiral  coiling  of  the  stem,  and  in  the  Peanut,  which 
draws  it  underground.  Others,  which  have  seeds  scattered  by 
wind,  greatly  elongate  the  stalks  of  the  seed  pods  during  ripening, 
thus  raising  them  to  a  position  more  exposed  ;  it  is  thus  in  the  Dan- 
delion. Some  seeds  become  attached  firmly  to  moist  ground  by 
aid  of  a  mucilaginous  substance  formed  from  their  coats  by  con- 
tact with  dampness,  though  the  advantage  thereof  is  not  clear, 
unless  the  attachment  aids  the  light  weight  of  the  seed  in  provid- 
ing a  resistance  permitting  the  root  to  be  forced  more  readily  into 
the  ground.  Again,  in  some  pods  the  seeds  will  not  all  germinate 
the  same  year,  even  under  perfect  conditions,  but  some  require  a 
year  longer  than  others,  —  thus  ensuring  the  perpetuation  of  the 
plants  even  if  all  the  seedlings  of  one  year  are  destroyed  by  any 


How  Plants  Secure  Change  of  Location  401 

calamity.  Some  seed  pods,  by  action  of  hygroscopic  mechanisms, 
open  only  when  the  weather  is  favorable  for  the  particular  mode 
of  dissemination  of  their  seeds,  whether  this  requires  wetness  or 
dryness.  And  there  are  yet  other  disseminational  adaptations, 
some  real,  some  accidental,  some  imaginary,  described  in  the 
works  of  Kerner  and  of  others  already  referred  to  in  the  foregoing 
pages.* 

When  we  view  as  a  whole  the  results  attained  through  these 
modes  of  dissemination,  and  note  how  wide  is  the  spread  some 
plants  have  secured  through  the  world,  it  becomes  plain  that  in 
the  long  run  the  sum  total  of  the  accomplishments  of  the  non- 
locomotive  plants  in  this  regard,  is  no  whit  inferior  to  that  of  the 
highly-locomotive  animals,  if  not  indeed  markedly  superior 
thereto.  This  shows  the  efficiency  of  the  dissemination  methods, 


*  Dissemination,  dealing  with  prominent  and  highly  developed  adaptations,  has 
always  been  one  of  the  favorite  topics  of  ecological  study;  and  it  affords  valuable 
material  alike  for  amateur  investigation,  for  student  themes,  and  for  popular  scientific 
articles  in  the  illustrated  magazines.  In  the  hope  that  the  reader  may  wish  to  follow 
this  subject  more  deeply  than  my  limits  allow,  I  add  here  the  titles  of  the  principal 
accessible  works  upon  it.  The  foundation  work  is  Hildebrand's  Die  Verbreitungsmittel 
der  Pflanzen  (Leipzig,  1873),  an  admirable,  but  all  too  brief  a  treatise,  which,  un- 
fortunately, has  never  been  translated.  There  is  a  wonderfully  clear  and  well- 
illustrated  account  in  Kerner's  Natural  History  of  Plants  (translated  by  Oliver, 
New  York,  Henry  Holt  &  Co.,  1895).  One  of  the  best  synopses,  illustrating  the 
striking  cases,  is  Beals'  excellent  little  book,  Seed  Dispersal  (Boston,  Ginn  &  Co., 
1898),  while  much  briefer  though  good  are  Weed's  Seed  Travellers  (Boston,  Ginn  & 
Co.,  1898)  and  a  chapter  in  Lubbock's  Flowers,  Fruits,  and  Leaves  (London,  The 
Macmillan  Co.,  1886).  Of  articles  accessible  in  magazines  the  best  are  Folsom's 
Adaptations  of  Seeds  and  Fruits,  in  Popular  Science  Monthly,  1893,  218,  and  es- 
pecially Ridley's  Dispersal  of  Seeds  by  Birds,  in  Natural  Science,  Vol.  8,  1896,  186, 
one  of  the  very  best  discussions  of  this  subject  anywhere  in  print.  And  of  course, 
there  is  a  host  of  special  papers  of  all  degrees  of  technicality  in  the  various  scientific 
magazines.  Considering  the  attractiveness  of  the  subject,  it  is  very  remarkable  that 
nobody  has  yet  undertaken  to  prepare  a  modern  cyclopedic  work  upon  it,  something 
comparable  with  the  books  we  possess  for  cross  pollination;  and  I  commend  this 
subject  to  any  ambitious  young  naturalist  among  my  readers,  warning  him  that  the 
task  is  vast  and  will  take  him  nearly  a  lifetime,  but  assuring  him  that  it  offers  an 
opportunity  for  just  such  a  distinctive  and  useful  piece  of  work  as  most  men  find  the 
greatest  satisfaction  in  doing.  There  is  not  in  science  any  kind  of  a  book  that  is  so 
lasting  in  value  as  this,  excepting  only  the  one  which  presents  material  wholly  new. 


402  The  Living  Plant 

while  illustrating  with  new  force  the  fact  that  the  race  is  not  al- 
ways to  the  swift. 

Finally,  it  is  important  to  note  that  the  ways  in  which  plants 
thus  secure  transport  for  their  seeds,  are  closely  analogous,  if  not 
identical,  in  principle  with  those  by  which  man  supplements  his 
own  feeble  powers  of  locomotion.  He  cannot  swim  far,  but  he 
can  provide  suitable  appliances  to  make  the  winds  carry  him  over 
the  broad  ocean;  he  cannot  fly  at  all,  but  he  can  place  suitable 
appliances  in  front  of  certain  explosive  forces  and  make  these 
drive  him  triumphantly  through  the  air.  The  difference  between 
the  Dandelion  plume  and  the  ship's  sail,  or  between  the  explosive 
capsules  of  Witch  Hazel  and  the  aeroplane  engine,  is  merely  one 
of  detail  and  degree.  Man  and  plant  are  doing  the  same  thing  in 
essentially  the  same  way,  the  chief  difference  being  that  man 
knows  what  he  is  doing  while  the  plant  does  not.  The  belief  that 
seed-wing  and  ship-sail  have  been  developed  in  ways  that  are 
fundamentally  the  same  helps  to  explain  what  I  meant  in  the 
chapter  on  Protoplasm  when  I  said  that  all  protoplasm  can  think. 


CHAPTER  XVI 

THE  METHOD  OF  ORIGIN  OF  NEW  SPECIES  AND 
STRUCTURES,  AND  THE  CAUSES  OF  THEIR  FITNESS 
TO  THE  PLACES  THEY  LIVE  IN 

Evolution  and  Adaptation 


F  the  various  aspects  which  Nature  presents  to  the 
intellect  of  man,  there  are  two  of  particular  promi- 
nence,— facts  and  explanations.  Of  these  the  greatest 
by  far  are  facts, — naked,  stark,  primitive,  elemental, 
cosmical  facts.  They  are  the  raw  material  of  science,  and  nothing 
can  replace  them.  But  when  one  has  made  himself  master  of  a 
goodly  number  thereof,  and  has  arranged  them  in  some  kind  of 
preliminary  classification,  he  soon  comes  to  crave  explanation  of 
the  remarkable  relations  they  are  sure  to  exhibit.  Explanation 
is  the  office  of  Philosophy,  and  there  is  a  Philosophy  of  Nature. 
The  phases  thereof  most  important  to  the  student  of  animals  and 
plants  concern  the  origins  of  their  multifarious  kinds,  of  their 
elaborate  structures,  and  of  their  remarkable  fitness  to  their 
surroundings.  There  was  a  time  when  none  of  these  were;  now 
they  all  are;  when  and  how,  in  the  interval  have  they  arisen? 
This  is  the  great  present  problem  of  philosophical  biology,  and 
one  which  the  reader,  fresh  from  his  contemplation  of  the  facts 
and  relationships  set  forth  in  the  preceding  pages,  is  now  pre- 
pared, and  I  hope  eager,  to  attack. 

There  are  two  great  explanations,  logically  and  historically,  of 
the  origin  of  species,  structures,  and  adaptations, — viz.,  Special 
Creation  and  Evolution.  The  doctrine  of  Special  Creation,  held 
almost  universally  down  to  a  half  century  ago,  maintained  that 

403 


404  The  Living  Plant 

every  kind  of  plant  and  of  animal,  with  every  one  of  its  manifold 
parts,  was  created  substantially  as  it  now  exists  at  some  definite 
time  in  the  past  by  the  act  of  an  omnipotent  and  omniscient 
Creator.  On  the  other  hand,  the  doctrine  of  Evolution,  now  held 
by  all  biologists  and  most  other  thinkers  as  well,  maintains  that 
each  species  of  plant,  and  each  one  of  its  structures,  has  been 
derived  by  gradual  modification  from  preexistent  and  simpler 
kinds,  which  in  turn  were  derived  from  yet  other  and  still  simpler 
kinds,  and  so  on,  in  an  unbroken  chain  of  descent  back  to  very 
ancient  and  very  simply-organized  ancestors,  whose  exact  mode 
of  origin  is  still  quite  unknown. 

It  is  now  a  long  time  since  it  was  thought  needful  to  present  in 
biological  courses  or  books  the  evidence  for  Evolution  against 
Special  Creation,  but  our  present-day  acceptance  of  Evolution  as 
almost  an  axiomatic  truth  involves  some  danger  of  leaving  our 
learners  in  ignorance  of  the  nature  and  force  of  the  evidence 
which  has  compelled  its  acceptance.  I  would  dearly  like  to 
present  this  evidence  to  the  reader  as  I  do  to  my  students,  but 
the  callous  incompressibility  of  paper  and  type  forbid;  and  it 
must  suffice  to  say  that  it  is  drawn  from  these  several  sources : — 
from  the  analogy  of  plant  and  animal  improvement  by  man  (soon  to 
be  considered  in  a  separate  chapter  on  Plant  Breeding),  whereby 
from  simple  wild  forms  of  both  animals  and  plants,  new  kinds, 
most  diverse  and  most  wonderful,  have  been  produced;  from  the 
results  of  classification,  which  show  that  the  kinds  of  plants  and 
animals  fall  naturally  into  an  arrangement  similar  to  that  estab- 
lished by  relationship  based  upon  descent  among  mankind,  some 
of  the  very  same  terms  indeed  (race,  tribe,  family)  being  used  in 
both  cases;  from  morphology,  which  shows  that  the  diverse  forms 
of  special  structures, — spines,  tendrils,  pitchers  and  so  forth, — 
are  all  modifications  of  simpler  preexistent  structures,  usually 
leaves,  stems  or  roots;  from  the  existence  of  gradations,  all  the 
way  up  in  regular  steps  from  the  very  simplest  kinds  of  plants  to 
the  most  complicated,  with  no  notable  gaps  or  missing  links  in 


Method  of  Origin  of  New  Species  and  Structures     405 

the  series;  from  fossils,  those  relics  of  ancient  plants  converted  to 
stone  and  preserved  in  the  rocks,  which  show  that  the  earliest 
plants  to  flourish  in  the  earth's  history  were  the  simpler  kinds, 
while  those  which  came  later  were  progressively  more  complex, 
and  the  very  highest  of  all  appeared  last;  from  geographical  dis- 
tribution, which  is  such  that  in  general  the  kinds  of  plants  most 
closely  related  are  found  nearest  together,  while  those  which  are 
farthest  apart  are  most  distantly  connected;  from  the  existence  of 
rudimentary  structures,  such  as  the  imperfect  stamens  in  irregular 
flowers,  or  the  appendix  in  man,  which  are  useless  to  their  present 
possessors,  but  are  useful  to  the  near  relatives,  and  hence  pre- 
sumably to  the  ancestors,  of  the  kinds;  from  embryology,  or  the 
course  of  development  of  the  individual  from  the  egg,  which 
often  exhibits  some  temporary  stages  quite  useless  to  the  develop- 
ing individual  but  useful  in  those  ancestors  which  the  form  must 
have  had  if  evolution  is  a  fact ;  and  from  yet  other  sources  which 
need  not  here  be  particularized.  In  all  of  these  directions  the 
phenomena  are  perfectly  explained  by  evolution,  but  present  well- 
nigh  insuperable  logical  difficulties  to  an  explanation  by  special 
creation.  Or,  the  case  can  be  stated  in  this  way, — if  evolution  be 
assumed,  then  the  facts  are  intelligible,  but  if  special  creation  be  as- 
sumed, then  they  are  enshrouded  with  inconsistency  and  mystery. 

I  may  venture  at  this  point  to  remind  the  reader,  though 
probably  the  caution  is  needless,  that  the  question  as  to  whether 
evolution  is  or  is  not  a  fact  is  a  purely  scientific  one,  to  be  judged 
by  purely  scientific  evidence,  tested  by  inexorable  scientific  logic. 
It  is  fatal  to  a  correct  judgment  upon  such  a  subject  to  approach 
it  with  preconceptions  or  prejudices  of  any  kind,  metaphysical, 
personal,  or  religious;  for  the  mind  of  man  is  so  organized  that 
whenever  it  seeks  evidence  for  some  favorite  belief,  it  has  no 
trouble  at  all  to  find  it.  To  him  who  puts  on  colored  glasses,  all 
things  look  of  that  color;  but  evolution  is  something  to  be  viewed 
only  in  the  purest  white  light  of  the  truth. 

But  while  evolution  is  accepted  as  a  fact  by  the  concensus  of 


406  The  Living  Plant 

present  biological  opinion,  biologists  differ  much  in  their  opinions 
as  to  the  method  by  which  it  has  been  effected, — for  of  course  the 
fact  or  non-fact  of  evolution  is  one  thing,  and  the  method  whereby 
it  has  been  brought  about  is  another.  Evolution  may  be  true 
and  yet  every  one  of  the  explanations  of  the  method  thereof, 
given  heretofore  by  scientific  men,  may  be  false.  These  explana- 
tions, however,  are  so  important  in  many  ways  that  we  must  now 
proceed  to  consider  them. 

Of  all  the  explanations  of  the  method  of  evolution,  the  greatest 
and  best  known  is  Darwin's,  embodied  in  his  principle  of  Natural 
Selection.  It  was,  indeed,  the  first  logically-satisfactory  explana- 
tion ever  given  of  the  way  in  which  evolution  may  have  been 
brought  about;  and,  because  it  was  logical,  it  enabled  thoughtful 
men  for  the  first  tune  to  believe  in  evolution  as  a  fact, — for  they 
could  not  believe  in  its  reality  so  long  as  they  could  not  under- 
stand how  it  might  have  been  effected.  Herein  consists  Darwin's 
greatest  service  to  science;  and  this,  moreover,  is  the  reason  why 
his  name  is  associated  with  evolution  so  closely  that  most  people 
regard  the  two  words  as  practically  synonymous.  And  the  case 
is  not  at  all  affected  by  the  fact  that  Natural  Selection  may  yet 
prove  not  to  be  the  real  explanation  of  evolution.  It  is  a  possible 
and  a  logically-adequate  explanation,  but  not  necessarily  on  that 
account  the  correct  one. 

So  important  is  this  principle  of  Natural  Selection,  historically 
as  well  as  scientifically,  that  we  must  now  consider  it  sufficiently 
to  make  its  significance  clear;  and  this  is  the  more  needful  be- 
cause it  is  commonly  misunderstood  even  by  many  of  those  who 
talk  much  about  it.  I  shall  try  first  to  present  the  subject  as 
I  think  that  Darwin  conceived  it,  giving  later  the  modifications 
introduced  by  subsequent  investigation. 

In  essence  Natural  Selection  is  a  deduction  from  the  inter- 
operation  of  five  factors,  all  of  which  are  familiar  to  observation, — 
viz.,  variation,  overproduction,  struggle  for  existence,  survival  of 
the  fittest,  heredity. 


Method  of  Origin  of  New  Species  and  Structures     407 

Variation. —It  is  a  matter  of  familiar  knowledge  that  all 
living  things,  or  the  structures  they  produce,  even  the  most 
closely-related,  are  different  from  one  another, — to  such  a  degree, 
indeed,  as  to  justify  the  common  saying  that  no  living  thing 
is  exactly  like  any  other  living  thing.  The  differences,  or  vari- 
ations, affect  every  possible  feature, — size,  form,  color,  texture, 
etc.,  and  occur  in  every  possible  direction;  and  some  of  them 
at  least  are  inherited  from  the  parents  and  transmissible  to 
offspring. 

Over-production. —All  living  beings  possess  a  power  of  repro- 
duction not  only  sufficient  to  replace  the  individuals  which  die, 
but  also  to  increase  greatly  their  numbers.  Moreover,  the  rate  of 
increase,  in  even  the  slowest  breeding  forms,  is  surprisingly  rapid, 
while  with  most  kinds  it  is  enormously  so.  Thus,  a  plant  which 
produces  only  ten  seeds  a  year  (and  few  produce  so  very  small  a 
number),  would  have  one  billion  descendants  within  ten  years, 
and  would  soon  cover  the  earth  to  the  exclusion  of  all  others  could 
its  increase  proceed  without  hindrance. 

Struggle  for  Existence. — Although  every  kind  of  plant  and  of 
animal  is  thus  tending  to  increase  enormously  in  numbers,  never- 
theless in  a  broad  way  those  numbers  remain  stationary  from  one 
generation  to  another.  Local  fluctuations  do  of  course  occur,  for 
some  kinds  of  plants  or  animals  are  on  the  way  to  extinction, 
while  others,  such  as  weeds  or  insect  pests,  have  periods  of  rapid 
expansion;  but  in  general  it  is  true  that  there  are  no  more  of  any 
particular  kinds, — lichens,  goldenrods,  thrushes  or  squirrels, — in 
a  given  region  one  year  than  another.  The  reason  thereof  is 
obvious  enough, — the  world  is  already  as  full  of  animals  and 
plants  as  there  is  food  or  room  for,  and  new  ones  can  find  a  place 
only  as  the  old  ones  die  out.  This,  then,  is  the  situation; — that 
while  great  numbers  of  plants  and  animals  are  born  into  the  world 
in  each  generation,  there  is  only  room  or  food  for  an  occasional 
one  of  the  number.  But  as  each  and  every  one  of  the  individuals 
thus  born  has  an  equal  right  and  impulse  to  survive  and  possess 


408  The  Living  Plant 

itself  of  the  scanty  room  and  food,  there  results  among  them  a 
constant  struggle  for  existence. 

Survival  of  the  Fittest. — In  this  struggle  for  existence  among  a 
great  many  individuals  of  which  few  can  survive,  what  determines 
which  those  few  are  to  be?  If  the  young  individuals  were  born 
all  alike  the  survival  would  obviously  be  determined  by  nothing 
but  chance;  but  in  fact  they  are  all  born  unlike,  and  among  the 
differences,  or  variations,  which  they  exhibit,  there  must  happen 
to  be  some  which  fit  their  possessors  better  for  the  conditions  of 
the  particular  struggle  in  hand  than  do  others;  and  such  better- 
fitted  forms  will  naturally  be  the  ones  to  succeed.  This  is  the 
survival  of  the  fittest.  Where  the  seedlings  of  spruce  trees  spring 
up  of  themselves  in  fields  that  are  abandoned,  it  comes  finally  to 
pass  that  a  few  tower  upward  in  full  vigor,  while  the  shade  under- 
neath them  is  almost  like  night  from  the  profusion  of  dead  stems 
of  the  unsuccessful, — the  ones  which  did  not  possess  the  variation 
of  most  rapid  upward  growth  to  possession  of  the  indispensable 
light. 

Heredity. — In  reproduction,  as  everybody  knows,  the  main 
features  of  the  parents  are  repeated  in  their  offspring,  or  are 
hereditary.  Now  this  is  true  also  of  at  least  a  part  of  the  varia- 
tions. Hence,  when  a  plant  or  an  animal  survives  by  virtue  of 
some  particular  advantageous  variation,  that  variation  is  likely 
to  be  repeated  in  its  offspring.  Meantime,  of  course,  the  unfit 
have  perished,  and  left  no  descendants.  The  whole  tendency, 
therefore,  is  towards  the  production  of  a  race  in  which  the  valuable 
variation  is  universal. 

it  will  now  be  evident  to  the  reader  that  these  five  factors  acting 
together  must  tend  to  cause  a  natural  selection,  and  hereditary 
fixation  in  each  generation,  of  the  fittest,  or  most  advantageous 
variations,  of  whatsoever  kind.  It  remains  to  consider,  and  this 
is  a  point  too  often  overlooked,  how  a  variation  can  accumulate 
and  become  intensified,  generation  after  generation,  until  it  forms 
a  well-marked  character  of  the  species.  This,  also,  is  easily  under- 


Method  of  Origin  of  New  Species  and  Structures     409 

stood  on  reflection.  For  not  only  do  the  offspring  of  the  parents 
preserved  by  possessing  a  fit  variation  inherit  that  variation,  but 
they  vary  in  regard  to  that  variation  itself.  Therefore  in  any 
generation,  while  some  of  the  individuals  will  inherit  the  variation 
about  like  the  parents,  a  few  will  vary  towards  a  greater  intensity 
thereof;  and  in  the  struggle  for  existence  in  this  generation, 
these  more  extreme  individuals  will  survive.  Then1  offspring, 
in  turn,  will  tend  to  resemble  them  in  possessing  the  greater 
degree  of  the  variation,  but  the  offspring  will  include  some  that 
vary  towards  an  even  higher  degree,  and  these  will  survive,  and 
so  on.  Thus  a  variation,  by  its  continued  selection  in  one  direc- 
tion generation  after  generation,  can  pile  up  until  it  produces  a 
large  and  visible  change  in  that  feature  of  the  plant.  But  the 
different  features  of  the  organism  are  so  closely  tied  together  that 
a  change  in  any  one  always  involves  some  others,  while,  moreover, 
selection  may  be  operating  upon  more  than  one  feature  of  a  plant 
at  a  time.  Thus  the  accumulation  of  variations  gradually  make 
their  possessor  look  distinctly  different  from  the  original  ancestors; 
and  when  that  point  is  reached  we  call  it  a  new  species,  especially 
if,  as  usually  but  not  invariably  happens,  the  intermediate  and 
less  well  adapted  forms  have  died  out  in  competition  with  the 
better.  The  process  is  represented  in  operation,  with  increase 
in  size  assumed  as  the  advantageous  variation,  in  the  accom- 
panying diagram  (figure  172).  This  process  of  progressive  adapta- 
tion would  continue  until  the  species  theoretically  has  become  as 
perfectly  adapted  as  possible  to  the  selective  conditions;  but  in 
fact  such  stability  would  never  be  reached  since  the  conditions 
themselves,  like  all  the  rest  of  the  world,  are  in  continual  altera- 
tion. And  such  is  THE  ORIGIN  OF  SPECIES  BY  MEANS  OF  NATURAL 

SELECTION,  OR  THE  PRESERVATION  OF  FAVORED  RACES  IN  THE 

STRUGGLE  FOR  LIFE,  in  the  words  of  the  title-page  of  Darwin's 
greatest  book. 

The  theory  of  Natural  Selection   explains  very  perfectly   not 
only  the  origin  of  new  structures  and  new  species,  but  also  the 


4io 


The  Living  Plant 


FIG.  172. — A  diagram  to  illustrate  the  three  leading 
explanations  of  the  method  of  evolution. 

A  spherical  organism,  represented  by  the  circles,  is 
assumed  to  be  living  under  conditions  where  in- 
crease of  size  is  advantageous.  The  course  of 
evolution  is  condensed  to  eleven  generations,  be- 
ginning on  the  left  and  running  up  to  the  right. 
The  dotted  lines  show  the  connection  between 
parents  and  offspring,  and  the  shading  in- 
dicates extermination  of  the  less  fit. 

The  upper  figure  represents  the  operation  of 
natural  selection.     Two  offspring  are  as-  ,/_ 
sumed  in  each  generation  of  which 
one  varies  to  a  size  larger  than 
the  parent,   the  other  remaining 
the  same. 

The  second  figure  represents 
the   operation  of  transmis- 
sion of  acquired  char- 
acters.     As   all    the 
offspring    are     alike 
from     this 
point    of 
view,  only 
one  of  each 
generation 


is    shown.     The 
adult     individuals 
are  assumed  to  ac- 
quire a  larger  size 
under  stimulation, 
and    to     transmit 
that  larger  size  to 
their  offspring. 
The  lower  figure  rep- 
resents the  operation  of  mutation.    The  indi- 
viduals are  substantially  alike  for  a  number 
of  generations,  then  suddenly  give  origin  to 
a  larger  type,  which  persists  unchanged  for 
a  time,  only  to  give  origin  to  a  still  larger,  and 


cause  of  their  adaptations  to  their  surroundings.    It  will  be  evident 
to  the  reader  that  the  principle,  while  logically  strong,  is  highly 


Method  of  Origin  of  New  Species  and  Structures     411 

hypothetical;  and,  needless  to  say,  mankind  has  not  yet  seen  the 
natural  evolution  of  a  species  by  this  process.  It  has  a  weakness 
in  the  fact  that  all  of  its  reasoning  is  on  the  assumption  "other 
things  being  equal,"  whereas  in  fact  the  innumerable  other  things 
rarely  are  equal.  It  has  a  strength,  on  the  other  hand,  in  the  fact 
that  the  kind  of  artificial  evolution  effected  by  man  in  the  produc- 
tion of  new  kinds  of  animals  and  plants,  uses  precisely  and  solely 
the  same  method  of  selection  and  preservation  of  variations. 
There  is,  however,  this  notable  difference  between  the  products 
of  artificial  and  natural  selection,  that  the  former  tend  always  to 
revert  back  towards  their  former  condition,  while  apparently  the 
latter  do  not;  and  to  many  observers  this  difference  seems  fatal  to 
any  support  of  natural  by  artificial  evolution.  It  may  be,  how- 
ever, that  the  time  element  in  the  process  is  important,  and  that 
the  comparative  rapidity  with  which  man  makes  his  new  kinds 
does  not  allow  the  new  characters  enough  time  to  " set."  If  one 
keeps  a  band  of  rubber  stretched  only  a  brief  time  it  springs 
back  to  its  old  shape;  if  longer,  only  partly;  if  long  enough,  not 
at  all! 

It  is  difficult  for  anyone,  and  impossible  for  me,  to  think  at 
much  length  about  Natural  Selection  without  recalling  its  great 
author.  Science  hath  her  heroes  no  less  than  war,  and  Darwin  was 
one  of  our  noblest.  An  Englishman,  born  in  1809  to  singular  good 
fortune  in  material  things,  and  fortunate  in  the  influences  which 
molded  his  intellectual  life,  he  came  slowly  to  his  great  concep- 
tion, which  he  first  published  when  he  was  fifty  years  old.  This 
was  in  his  book  The  Origin  of  Species,  which  by  common  consent 
is  agreed  to  have  exercised  a  more  profound  effect  than  any 
other  secular  book  upon  human  thought.  It  is  difficult  for  us 
in  these  more  liberal  days  to  comprehend  the  bitterness  of  the 
opposition  which  his  support  of  evolution  aroused,  partly  among 
the  older  naturalists  but  chiefly  among  those  who  imagined  that 
the  foundations  of  religion  were  endangered.  But  through  all  the 
storm  he  stood  steadfast, — calm,  just,  and  magnanimous,  even 


412  The  Living  Plant 

though  to  his  other  great  provocations  was  added  the  torment  of 
chronic  ill-health.  Of  him  his  friend  Huxley  has  said, — "The 
more  one  knew  of  him,  the  more  he  seemed  the  incorporated  ideal 
of  a  man  of  science."  Possessing  vast  speculative  powers,  he 
nevertheless  kept  his  imagination  in  touch  with  the  truth  by  in- 
cessant and  laborious  observation  and  experiment.  Yet  this 
greatest  of  all  naturalists  was  no  demi-god,  much  less  a  person 
abnormal  to  his  kind,  but  a  warm-hearted,  humanly-interested, 
honorable-souled  gentleman.  He  lived  to  see  the  complete 
triumph  of  his  life-work,  and  died  in  high  honor  in  1882  at  the  age 
of  seventy-three.* 

The  second  in  importance,  though  first  in  time,  of  the  great 
explanations  of  evolution  was  Lamarck's  principle  of  the  "trans- 
mission of  acquired  characters."  It  is  almost  the  exact  logical 
opposite  of  natural  selection,  and  the  life  of  its  author  contrasts 
almost  as  greatly  with  that  of  Darwin.  A  Frenchman,  born  in 
1744,  he  was  at  the  height  of  his  career  about  fifty  years  before 
Darwin,  as  Darwin  was  fifty  years  before  our  own  time;  and  it  is 
a  coincidence  of  no  little  interest  that  the  work  in  which  he  most 
fully  expounded  his  views  was  published  in  1809,  exactly  fifty 
years  before  the  Origin  of  Species.  But  Lamarck,  unlike  Darwin, 
failed  to  keep  his  imagination  checked  by  investigation,  and  his 
theories  in  close  touch  with  the  facts.  Therefore  he  had  the 
mortification  to  see  his  favorite  work  ignored  by  his  contempo- 
raries; and  he  died,  in  1829,  in  disappointment,  infirmity  and 

*  The  reader  will  wish  to  know  more  about  Darwin,  and  will  find  great  satisfaction 
in  a  study  of  his  Life  and  Letters  (one  of  the  great  biographies  of  literature)  by  his 
son  Francis.  In  that  work,  his  own  autobiography,  and  his  son's  reminiscences,  are 
of  first  interest,  but  the  most  charming  glimpses  of  his  character  are  given  by  his 
letters, — for  example,  that  written  to  his  wife  from  Moor  Park,  in  April,  1858,  and 
that  written  to  his  friend  Asa  Gray,  on  August  9,  1862.  And  the  reader  should  not 
fail  to  read  the  remarkable  obituary  of  Darwin  by  Huxley  in  Nature  for  April  27, 
1882, — doubtless  the  noblest  tribute  ever  paid  by  one  scientific  man  to  another. 
The  Origin  of  Species  is  not  an  easy  book  to  read,  nor  can  it  be  really  appreciated 
by  anyone  until  he  has  acquired  a  considerable  background  in  biological  knowledge: 
but  after  that  the  reasons  for  its  real  greatness  become  clearly  apparent. 


Method  of  Origin  of  New  Species  and  Structures     413 

poverty.  Yet  though  his  labors  were  seemingly  without  immedi- 
ate fruit,  they  were  of  great  service,  nevertheless,  in  awakening 
men's  minds  to  the  problems  and  possibilities  of  evolution,  and 
thereby  making  the  way  easier  for  Darwin. 

Lamarck's  theory  is  founded  on  two  factors, — the  alterability 
of  individuals,  and  the  hereditary  transmission  of  the  results 
thereof. 

The  Alterability  of  Individuals. — This  is  not  theory,  but  fa- 
miliar fact.  Everybody  knows  that  our  own  muscles,  with  their 
associated  blood  vessels,  nerves,  and  bones,  can  be  improved  in 
size  and  strength  by  exercise,  as  can  our  minds,  and  other  features 
of  our  being;  and  likewise  these  can  all  degenerate  through  disuse. 
And  so  it  is  throughout  the  animal  kingdom.  The  process  is  well 
understood:  the  use  of  the  part  serves  as  a  stimulus  which  leads 
the  organism  to  throw  more  material  and  energy  into  those  parts, 
precisely  in  the  manner  which  we  have  studied  already  in  the 
chapter  on  Irritability.  In  the  same  chapter  we  have  seen  also 
how  plants  can  alter  their  structure  under  stimulation.  Thus 
some  kinds  of  plants  can  develop  a  thicker  epidermis  under  the 
stimulus  of  dry  air;  some  trees  can 'apparently  build  stronger 
stems  under  stimulation  of  bending  by  the  wind ;  and  indeed  there 
seems  to  be  no  limit  to  the  directions  and  degrees  in  which  plants 
can  respond  structurally,  and  adjustively,  to  stimulation.  The 
structural  alterations  thus  produced  in  individuals,  are  called 
acquired  characters. 

Transmission  of  Acquired  Characters. — This  is  the  crucial  point 
in  the  theory  of  Lamarck.  He  held  that  characters  acquired 
during  the  life  of  the  individual,  as  described  above,  are 
transmitted  to  their  later  offspring,  which,  therefore,  exhibit 
larger  muscles,  or  finer  minds,  or  thicker  epidermis,  or  stronger 
stems,  than  would  have  been  the  case  had  the  parents  not  devel- 
oped those  features.  Lamarck  actually  uses  the  illustration  of 
the  blacksmith's  arm,  powerfully  developed  in  the  practice  of  his 
trade;  and  he  maintains  that  the  later-born  sons  of  the  smith  will 


414  The  Living  Plant 

have  more  powerful  arms  than  would  have  been  the  case  had 
their  father  adopted  some  less  strenuous  trade.  Most  of  our 
popular  beliefs  tend  to  the  same  end,  especially  as  to  moral  and 
intellectual  qualities;  for  it  is  commonly  supposed  that  the  finer 
mind  developed  in  an  individual  by  high  education,  or  the  de- 
generacy produced  by  submission  to  vice,  are  somehow  trans- 
mitted to  the  offspring.  If  a  feature  is  hereditary  for  one  genera- 
tion, however,  it  is  hereditary  for  more;  and  thus,  according  to 
Lamarck,  a  character  can  go  on  piling  up  generation  after  genera- 
tion until  it  reaches  a  degree  of  development  sufficient,  along  with 
associated  changes,  to  make  its  possessor  rank  as  a  new  species. 
Of  course,  on  this  principle,  all  individuals  born  into  the  world 
have  an  equal  chance  for  survival,  and  mere  chance  would  de- 
termine success.  This  method  of  evolution  is  illustrated  in  com- 
parison with  that  by  natural  selection  on  the  accompanying 
diagram  (figure  172). 

The  Lamarckian  explanation  of  evolution  has  a  great  merit  in 
its  simplicity,  but  has  the  fatal  defect  that  the  crucial  trans- 
mission of  acquired  characters  is  not  confirmed  either  by  ordinary 
observation,  or  by  any  experiment  which  has  been  devised  to 
test  it.  Moreover,  that  gigantic  system  of  experiment  always  in 
progress  in  plant  and  animal  improvement  by  man  has  failed  to 
yield  one  fact  in  its  support.  Furthermore,  the  phenomena 
which  apparently  are  the  strongest  in  its  favor  can  be  explained 
more  simply  in  other  ways.  Thus,  the  blacksmith's  sons,  it  is 
true,  tend  to  have  stronger  arms  than  ordinary  men;  but  this 
need  not  mean  that  they  inherited  the  stronger  arm  acquired  by 
the  father,  but  only  that  they  inherited  the  same  robust  proto- 
plasm which  enabled  the  father  to  become  a  successful  smith.  So 
the  children  of  highly  educated  parents  are  apt  to  be  bright,  not 
because  they  inherited  the  educated  minds  of  the  parents,  but 
because  they  inherited  the  finer  quality  of  mind-protoplasm 
which  made  high  education  in  the  parents  a  possibility;  and  so 
with  the  children  of  tuberculous  parents,  who  inherit  not  the 


Method  of  Origin  of  New  Species  and  Structures     415 

tuberculosis,  but  the  weak  lungs  which  render  tuberculosis  pos- 
sible. Taken  as  a  whole,  therefore,  the  evidence  we  possess  upon 
the  subject  does  not  tend  to  support  the  Lamarckian  theory. 

The  contrast  between  the  theories  of  Darwin  and  of  Lamarck 
is  given  the  sharpest  definition  by  the  work  of  Weismann,  an 
eminent  German  zoologist  still  living.  Darwin  himself,  while 
convinced  that  natural  selection  was  the  leading  factor  in  effect- 
ing evolution,  was  inclined,  especially  in  later  life,  to  admit  some 
transmission  of  acquired  characters;  and  indeed  he  actually  in- 
vented a  special  theory  (called  pangenesis,  a  flow  of  tiny  solid 
particles  from  all  parts  of  the  body  to  the  germ  cells),  to  explain 
a  possible  mode  of  its  operation;  but  his  follower  Weismann  stood 
for  Natural  Selection,  pure  and  simple,  as  against  the  rival  theory, 
and  even  invented  an  ingenious  conception  to  explain  the  natural- 
ness of  the  operation  of  the  one  and  the  impossibility  of  the  other. 
In  brief,  he  held  that  there  are  two  kinds  of  protoplasm  in  each 
animal, — one  reproductive,  the  germ  plasm,  confined  to  the  eggs 
and  the  sperm  cells,  and  the  other  the  body  plasm,  making  up  all 
the  rest  of  the  organism.  Now  the  fertilized  egg-cell,  from  which 
the  new  individual  grows,  is  obviously  germ  plasm.  As  it  grows 
and  develops,  a  part  of  the  resultant  cells  keep  on  being  germ 
plasm,  which,  however,  remains  latent  until  the  animal  is  adult, 
while  the  remainder  of  the  cells  develop  into  body  plasm,  which 
grows  immensely  and  comes  ultimately  to  surround,  protect  and 
nourish  the  embedded  germ  plasm.  Then  when  the  time  for 
reproduction  has  arrived,  it  is  always  the  latent  germ  plasm, 
never  the  body  plasm,  which  builds  the  new  egg-cells  and  sperm 
cells,  whose  union  starts  another  individual  in  the  same  way  as 
before.  Thus  germ  plasm  produces  body  plasm,  but  body  plasm 
never  produces  germ  plasm.  Hence  the  germ  plasm  is  potentially 
immortal,  keeping  on  as  one  continuous  line  of  tissue  from  genera- 
tion to  generation,  while  the  body  plasm  is  mortal,  made  anew  in 
each  generation  and  perishing  utterly  therewith.  The  matter  can 
be  expressed  also  in  this  manner, — that  the  germ  plasm  forms  a 


416  The  Living  Plant 

kind  of  continuous  axial  thread  upon  which  the  body  plasm  is 
strung  at  intervals  like  beads  on  a  string,  except  that  we  have  to 
imagine  the  beads  as  growths  from  the  string!  On  this  theory 
the  germ  plasm  is  the  essential  protoplasmic  basis  of  the  race,  and 
the  body  is  simply  an  organ  which  it  builds  to  secure  its  own 
nutrition  and  protection.  Now  it  is  obvious  that  any  variation 
which  originates  in  the  germ  plasm  can  show  itself  in  all  of  the 
succeeding  germ  plasm,  and  also  in  all  of  the  bodies  which  grow 
out  therefrom;  on  the  contrary,  any  variation,  or  other  feature, 
including  an  acquired  character,  which  originates  in  the  body 
plasm,  must  perish  with  that  body  and  cannot  affect  the  bodies 
which  come  after,  unless  it  can  go  round  through  the  germ  plasm, 
for  which  no  mechanism  is  known  to  exist,  excepting  possibly 
that  mentioned  in  a  paragraph  to  follow.  This  theory  of  Weis- 
mann's  explains  very  perfectly  an  evolution  by  natural  selection 
of  innate  (in-born  or  germ-plasmic)  variations,  and  also  supplies 
a  reason  for  the  non-transmissibility  of  acquired  characters.  It 
shows  how  children  can  exhibit  cultured  minds  or  large  muscles 
like  their  parents  without  inheriting  the  results  of  their  parents' 
culture  or  exercise,  for  while  the  results  of  culture  or  exercise  are 
confined  to  the  body  plasm,  and  perish  when  it  does,  the  capacity 
to  develop  the  results  depends  on  the  constitution  of  the  germ 
plasm  which  parents  and  children  share  alike.  The  bodies  of 
children  resemble  the  bodies  of  their  parents,  therefore,  not  be- 
cause the  former  are  derived  from  the  latter,  but  because  both 
are  derived  from  the  same  source.  This  ingenious  theory  of 
Weismann's  has  not  been  confirmed  by  further  research  so  far  as 
its  physical  basis  in  the  two  kinds  of  protoplasm  is  concerned,  and 
it  never  applied  well  to  plants,  which  seem  very  clearly  at  times 
to  create  germ  plasm  out  of  body  plasm ;  but  I  give  it  this  much  of 
our  attention  because  all  recent  research  is  tending  to  confirm  the 
correctness  of  its  central  principle,  which  stands  perfectly  when 
expressed  in  this  way, — that  body  characters  are  derived  from 
germinal  determinants,  which  in  turn  are  derived  from  preceding 


Method  of  Origin  of  New  Species  and  Structures     417 

germinal  determinants  in  a  continuous  line,  but  never  from  body 
characters.  And  the  fallacious  physical  basis  given  his  theory  by 
Weismann  has  been  replaced  by  a  secure  one  supplied  by  Mendel- 
ian  studies,  presently  to  be  considered.  Indeed,  the  modern  con- 
ceptions of  heredity,  based  on  Mendelian  results,  is  a  veritable 
reincarnation  of  the  central  feature  of  Weismannism. 

Before  leaving  this  part  of  the  subject  it  is  needful  to  say  that 
such  evidence  as  does  seem  to  favor  a  transmission  of  acquired 
characters  is  found  in  connection  with  certain  diseases.  The  cases 
seem  to  hinge  upon  chemical  changes  produced  in  the  blood,  which, 
circulating  and  diffusing  throughout  the  whole  body,  can  thus 
reach  the  germ  cells  and  through  them  the  next  generation.  On 
this  basis,  any  acquired  character  which  affects  the  chemistry  of 
the  blood  could,  theoretically,  be  transmitted  to  the  germ  plasm 
and  the  next  generation,  although  changes  which  are  simply  of  a 
physical  or  mechanical  nature  could  not.  We  have  a  close  anal- 
ogy in  the  relation  of  scion  characters  to  stock  characters  in 
grafting,  already  considered  (page  350);  for  it  appears  to  be 
generally  true  that  characters  which  are  dependent  upon  the  sap 
can  be  extended  or  ''transmitted"  from  the  scion  to  the  stock,  and 
vice  versa,  while  characters  which  are  dependent  upon  the  pro- 
toplasm are  confined,  on  the  contrary,  strictly  to  scion  or  stock 
respectively.  This  principle  of  chemical  transmission  may  yet 
prove  to  be  important  in  evolution,  and  may  rehabilitate  Darwin's 
theory  of  pangenesis  on  a  new  basis;  and  it  accords  with  the  tend- 
ency of  all  modern  research  to  reduce  natural  phenomena  to  a 
chemical  foundation.  Indeed,  some  students  of  the  subject  have 
suggested  that  the  chromosomes,  those  carriers  of  heredity  in  the 
nuclei  of  cells,  are  simply  collections  of  enzymes,  each  of  which 
controls  some  single  process  of  development  in  the  new  individual. 

The  study  of  the  problems  of  evolution  exhibits  three  separate 
epochs.  The  first  was  that  of  speculation  from  impressions,  cul- 
minating in  the  theories  of  Lamarck.  The  second  was  that  of 
induction  from  observation,  inaugurated  and  carried  to  highest 


4i8  The  Living  Plant 

perfection  by  Darwin.  The  third  is  that  of  test  through  experiment, 
of  which  we  are  witnessing  the  very  beginning.  Its  great  leader 
is  de  Vries,  an  eminent  Hollander  still  active  in  scientific  service. 
Some  twenty  years  ago  de  Vries  noticed  in  Holland  a  certain 
weed, — an  American  Evening  Primrose,  called  (Enothera  Lamarck- 
iana  (note  the  coincidence  of  name!), — which  showed  such  re- 
markable phenomena  of  variation  that  he  brought  some  of  the 
plants  into  his  botanical  garden  where  he  could  study  their  be- 
havior with  exactness.  The  result  was  remarkable  indeed,  for 
he  saw  new  kinds  originating  before  his  eyes,  not  by  any  slow 
process,  but  the  fastest  that  is  physically  possible, — viz.,  in 
one  step  from  parent  to  offspring.  When  seeds  were  taken  from 
the  ripe  pods  of  (Enothera  Lamarckiana,  and  planted  with  pre- 
cautions which  precluded  all  possibility  of  error,  most  of  the  seeds 
grew  into  plants  like  the  parents;  but  some  grew  into  a  much 
smaller  kind,  others  into  a  much  larger  kind,  and  yet  others  into 
other  kinds,  differing  in  other  respects  (figure  173).  Thus  from 
the  parent  species  several  daughter  kinds  were  produced,  and  not 
once  alone  but  regularly  generation  after  generation.  The  new 
kinds  do  not  differ  much  from  the  parent,  but  enough  to  enable 
trained  botanists  to  distinguish  them  with  certainty;  and  more- 
over they  differ  not  in  one  but  in  a  great  many  features.  The 
individuals  of  any  one  of  these  new  kinds  exhibit  minor,  or  fluc- 
tuating, variations  among  themselves  it  is  true;  but  they  preserve 
throughout  a  sufficiency  of  definite  characters  in  common.  Fur- 
thermore, and  this  is  a  matter  of  the  very  greatest  importance, 
when  seeds  of  each  of  the  daughter  kinds  were  planted  by  them- 
selves, they  reproduced  each  their  own  kind,  and  that  not  alone 
for  one  generation,  but  for  several,  and  indeed  for  as  many  as 
time  has  allowed  since  their  discovery.  Finally,  the  same  ex- 
periments have  been  repeated  elsewhere,  and  with  identical  re- 
sults. There  seems  no  doubt,  therefore,  that  this  species  of 
Evening  Primrose  is  actually  giving  off  several  new  kinds  year 
after  year,  and  kinds  which  reproduce  themselves  permanently. 


Method  of  Origin  of  New  Species  and  Structures     419 

Such  new  kinds  were  supposed  by  de  Vries  to  be  species,  of  an 
ultimate  or  elementary  sort,  and,  in  reference  to  their  mode  of 
origin  he  designated  them  mutants,  while  the  parent  species  he 
described  as  being  in  mutation.  As  to  the  exact  relation  of  muta- 
tion to  evolution,  that  was  supposed  by  de  Vries  to  be  this, — that 
mutants,  or  elementary  species,  and  not  single  variations,  are 


FIG.  173. — Groups  of  the  mutants  of  (Enothera,  growing  in  de  Vries'  experimental  garden 
at  Amsterdam.  The  parent  species,  (E.  Lamarckiana,  is  the  single  one  on  the  extreme 
left.  In  one  group  two  flowers  are  covered  with  bags  for  experimental  purposes.  Ob- 
serve the  distinctness  of  the  groups  from  one  another,  in  conjunction  with  a  certain 
amount  of  variability  within  each  group.  (Photographed  from  a  colored  picture  in 
de  Vries'  book,  The  Mutation  Theory,  Vol.  II.) 


the  material  upon  which  selection  works.  Darwin  thought  the 
basal  variations  were  mostly  single,  finely-graded,  and  more  or 
less  unstable,  while  de  Vries  offers  instead  collections  of  varia- 
tions large,  definite  and  permanent;  but  otherwise  their  views  are 
in  full  harmony,  both  agreeing  that  natural  selection  is  the  final 
factor  which  determines  survival.  Like  variations,  the  mutations 


420  The  Living  Plant 

which  happen  to  be  adaptive  would  be  preserved  in  the  struggle 
for  existence,  while  the  unadapted  would  perish.  Then,  according 
to  the  theory,  after  a  time  the  surviving  species  would  mutate 
again,  and  the  fittest  of  its  mutants  would  survive,  and  so  on. 
Thus  would  the  species  be  kept  adapted  approximately  to  en- 
vironments, while  evolution  would  take  place  in  a  series  of  short 
abrupt  steps  separated  by  long  pauses, — a  condition  illustrated 
in  the  lower  part  of  our  diagram  (figure  172).  De  Vries'  view  of 
evolution  has,  moreover,  one  marked  advantage  over  Darwin's 
in  explaining  the  existence  of  species  and  structures  which  bear 
no  adaptive  relations  to  the  environment,  such  for  example  as 
the  wonderfully  diverse  forms  and  markings  of  the  Diatoms;  for 
such  features  can  originate  and  reach  a  considerable  degree  of 
development  by  mutation,  if  not  cut  off  by  natural  selection, 
while  on  Darwin's  view  only  those  things  which  were  adaptive 
had  any  chance  of  a  considerable  development. 

The  great  interest  of  de  Vries'  work  at  once  stimulated  a  wide 
search  for  other  examples  of  mutation  in  both  animals  and  plants; 
and  a  very  few  other  cases  have  been  discovered.  Attempts  have 
also  been  made  to  set  species  into  mutation  experimentally,  but 
with  only  dubious  success.  It  is  thus  plain  that  species  in  muta- 
tion are  very  few, — which  fact  is  explained  by  de  Vries  on  the  sup- 
position that  species  display  short  periods  of  mutation  separated 
by  long  periods  of  quiescence. 

But  though  species  in  mutation  are  undoubtedly  rare,  species 
apparently  identical  with  mutants  have  been  found  to  be  common, 
though  there  is  no  evidence  as  to  how  they  have  arisen.  The  more 
intensive  studies  of  the  past  few  years  have  shown  that  species 
formerly  considered  single  are  in  reality  aggregates  of  dozens  or 
hundreds  of  these  mutant-like  species,  which  have  now  become 
so  prominent  in  scientific  literature  that  they  have  attained  to 
a  distinctive  designation  of  their  own,  viz.,  elementary  species  or 
biotypes.  Thus  the  little  "Ladies  Tobacco"  of  our  earliest  spring 
fields,  thought  by  the  acutest  observers  of  the  last  generation  to 


Method  of  Origin  of  New  Species  and  Structures     421 

represent  but  one  species  (viz.,  Antennaria  plantaginifolia) ,  has 
been  found  to  include  a  dozen,  all  perfectly  distinct,  permanent, 
and  recognizable  by  good  observation;  the  Brambles  and  some 
Grasses  have  been  claimed  to  include  not  dozens  of  species  but 
hundreds;  and  the  Hawthorns  of  America  have  already  been 
described  to  the  number  of  over  a  thousand,  with  no  end  to  the 
trouble  as  yet.  The  same  thing  is  true  also  of  cultivated  plants, 
and  strikingly  so  of  the  grains.  A  field  of  Indian  Corn,  for  ex- 
ample, has  been  found  to  consist  not  of  one  species,  as  we  used  to 
suppose,  but  of  dozens  of  biotypes,  or  elementary  species,  crossing 
and  hybridizing  greatly  it  is  true,  but  capable  of  separation  and 
ultimate  pure  breeding  each  by  itself.  It  is  important  to  remem- 
ber, however,  that  the  fact  of  the  existence  of  these  elementary 
species  is  quite  independent  of  the  question  as  to  their  origin;  and 
many  of  those  who  have  had  most  to  do  with  their  discovery 
doubt  whether  they  have  arisen  by  mutation,  though  de  Vries,  of 
course,  believes  that  they  did. 

There  is  one  other  great  name  associated  with  evolution,  even 
though  somewhat  indirectly,  and  that  is  Mendel,  whose  dis- 
coveries in  the  particular  field  of  heredity  are  exerting  a  profound 
influence  upon  present-day  evolutionary  thought.  We  have 
already  discussed  his  work  in  our  chapter  on  Reproduction,  and 
need  only  summarize  here  the  points  of  importance  to  our  im- 
mediate subject.  They  are  these: — 

First,  each  individual  organism,  animal  or  plant,  is  an  aggre- 
gate, or  mosaic,  as  it  were,  of  a  definite  number  of  characters 
each  of  which  is  represented  by  a  determiner  or  unit  in  the  germ 
cells  from  which  it  has  developed.  These  characters  are  thou- 
sands in  number  in  the  higher  organisms,  fewer  in  the  simpler,  and 
include  all  kinds  of  features  of  structure,  form,  size,  color,  etc. 
Thus  eye-color  in  man,  and  the  number  of  rows  of  grains  on  an 
ear  in  Corn,  are  such  unit  characters. 

Second,  every  germ  cell,  whether  egg-cell  or  sperm-cell,  con- 
tains one  complete  set  of  units  capable  of  reproducing  all  the 


422  The  Living  Plant 

characters  of  the  organism,  but  never  any  duplicate  units.  On 
the  other  hand,  the  fertilized  egg-cell,  and  every  cell  of  the  body 
subsequently  arising  therefrom,  contains  a  duplicate  or  double 
set,  one  from  each  parent,  from  which  selection  has  to  be  made 
during  the  development  of  the  organism;  and  this  double  set  pre- 
vails until  the  new  formation  of  the  germ  cells,  each  of  which 
in  the  "reduction  division"  receives  but  a  single  set. 

Third,  while  each  one  of  these  newly-formed  germ  cells  contains 
a  complete  set  of  units,  these  are  partly  derived  from  one  of  the 
parents,  and  partly  from  the  other.  Moreover,  in  any  given  germ 
cell,  the  paternal  and  maternal  units  are  mixed  in  the  most  com- 
plicated manner,  and,  furthermore,  hardly  any  two  germ  cells  can 
be  found  with  the  same  combination.  Consequently  when  the 
unions  of  these  germ  cells  in  reproduction  are  left  wholly  to 
chance,  as  Mendel's  results  prove  that  they  are,  then  the  most 
diverse  possible  combinations  of  paternal  and  maternal  charac- 
ters must  result,  even  among  close-fertilized  kinds  such  as  many 
plants  are,  while  the  complications  are  proportionally  greater 
among  cross-fertilized  beings,  like  mankind.  We  have  thus  the 
explanation  of  the  very  familiar  fact  that  no  brothers  or  sisters 
are  ever  found  who  exhibit  the  same  combination  of  characters  of 
father  and  mother, — even  the  case  of  identical  twins  being  no  real 
exception,  since  these  are  known  to  arise  from  the  splitting  of  one 
fertilized  egg-cell. 

We  have  now  brought  our  subject  quite  down  to  our  own  days, 
which  are  distinguished  by  extreme  activity  in  experiment.  It  is 
not  possible,  however,  to  estimate  as  yet  the  value  of  the  results 
that  seem  to  be  accruing  therefrom.  We  lack  perspective,  of 
course;  and  moreover  the  conclusions  have  not  yet  received  the 
thorough  critical  testing  which  is  essential  to  establish  that 
"impersonal  validity,"  without  which  they  cannot  rank  as 
scientific  knowledge.  But  I  shall  add  here  a  synopsis  of  the 
principal  matters  which  seem  to  be  crystallizing  out  from  these 
studies. 


Method  of  Origin  of  New  Species  and  Structures     423 

First,  exact  studies  on  variation  seem  to  show  that  the  fortu- 
itous variations  of  Darwin  are  of  two  distinct  kinds  or  classes. 
One  class,  now  called  fluctuating  variations,  includes  those  caused 
by  the  immediate  environment  acting  either  forcibly  (producing 
injury,  &c.),  or  through  stimuli  calling  out  irritable  responses; 
they  are  not  hereditary,  and  therefore  have  no  influence  in  evolu- 
tion. They  are  variations  of  the  body  plasm  only,  in  Weismann's 
sense.  The  reader  will  find  good  examples  of  variations  of  this 
type  in  the  differences  between  the  individuals  within  each  of  the 
mutation  groups  shown  in  figure  173.  The  other  class  includes 
those  that  are  inborn,  and  hereditarily  transmitted  from  genera- 
tion to  generation, — variations  of  the  germ  plasm,  in  Weismann's 
sense.  These  are  variations  upon  which  natural  selection  works. 
Their  origin  is  unknown,  but  they  are  related  if  not  identical  with 
mutations,  and  with  permutations  and  combinations  of  Mendelian 
unit  characters.  The  two  classes  of  course,  are  indistinguishable 
by  the  eye,  and  only  determinable  by  experiment. 

Second,  the  very  newest  studies,  announced  during  the  writing 
of  this  book,  appear  to  be  demonstrating  that  the  mutants  of 
Evening  Primrose  discovered  by  de  Vries,  are  simply  in  large  part 
the  separation  or  segregation  out  of  original  elementary  species 
which  hybridized  together  to  form  the  original  (Enothera  La- 
marckiana.  This  case  of  mutation,  therefore,  is  not  an  instance 
of  the  appearance  of  new  species,  but  simply  of  the  reappearance 
of  old  ones  temporarily  obscured  in  a  combination;  and  it  leaves 
unsolved  the  question  of  the  origin  of  elementary  species. 

Third,  all  the  recent  work  is  confirming  the  reality  of  the  exist- 
ence of  elementary  species  or  biotypes,  though  it  is  throwing  very 
little  light  on  their  origin.  Moreover,  and  here  is  a  most  important 
point,  it  is  showing  that  these  biotypes,  though  apparently  homo- 
geneous (and  therefore  forming  a  single  phcmotype),  are  in  reality 
composite,  since  they  embrace  a  good  many  Mendelian  com- 
binations (or  genotypes}.  But  it  is  not  worth  while  to  follow  these 
matters  further  at  present,  since  we  now  verge  close  to  the  firing 


424  The  Living  Plant 

line,  where  issues  are  doubtful.  It  must  suffice  to  say  that  our 
knowledge  of  these  subjects  is  in  process  of  active  extension  at 
this  moment. 

Fourth,  exact  study  devoted  to  determining  whether  the  selec- 
tion of  variations,  in  the  Darwinian  sense,  can  actually  produce 
a  new  species  have  given  very  largely  a  negative  result, — much 
evidence  tending  to  show  that  selection  simply  isolates  the  bio- 
types,  but  cannot  in  any  way  alter  them.  If,  however,  biotypes 
originate  from  other  biotypes,  as  it  seems  that  surely  they  must, 
then  the  method  of  evolution  would  be  substantially  that  im- 
agined by  de  Vries  for  his  mutants,  and  that  represented  in  our 
comparative  diagram  (figure  172).  Thus,  selection  would  still 
rank  as  the  great  decisive,  though  not  as  an  originating,  factor 
in  evolution.  As  to  adaptation,  that  still  stands  as  a  corollary  of 
any  kind  of  evolution  by  selection,  for  selection  imposes  a  step- 
by-step  development  in  touch  with  the  environment.  The  con- 
ception of  biotypes  is  wholly  consistent  therewith,  and  indeed 
helps  to  explain  some  of  the  peculiarities  of  adaptation, — es- 
pecially the  somewhat  loose,  clumsy,  or  generic  character  that 
most  adaptation  displays  in  conjunction  with  the  occasional 
existence  of  highly  exact  fitness.  In  general  in  Nature,  structure 
fits  function  about  as  well  as  a  man's  physique  fits  his  trade, — 
that  is,  always  in  a  general  way,  and  sometimes  very  exactly. 
We  cannot  expect  rigid  biotypes  to  fit  intergrading  environments 
any  more  than  we  can  expect  polygons  to  match  circles, — though 
with  some  many-sided  kinds,  the  correspondence  can  be  appre- 
ciably close.  But  it  is  perfectly  clear  that  the  first  great  problem 
of  present-day  experimental  evolution  is  the  determination  of  the 
origin  of  biotypes,  or,  to  be  exact,  of  the  variations  or  characters 
which  constitute  biotypes.  I  should  not  be  surprised  if  it  were  to 
turn  out  that  the  origination  of  new  characters  or  biotypes  is  a 
normal  function  of  organisms,  adaptively  acquired  by  them  pre- 
cisely as  any  other  physiological  function  has  been,  and  represents 
their  method  of  securing  survival  in  changing  environments.  It 


Method  of  Origin  of  New  Species  and  Structures     425 

is  a  voluntary  offering  of  material,  so  to  speak,  for  selection  to 
choose  from. 

Such  is  the  present  state  of  uncertainty  in  our  knowledge  of 
some  of  the  most  fundamental  matters  in  evolution.  The  truth 
we  shall  learn  later  through  intensive  study  and  experiment.  At 
many  places  the  world  over, — at  the  Desert  Botanical  Laboratory 
in  Arizona,  at  the  Station  for  Experimental  Evolution  on  Long 
Island,  at  many  Government  and  University  Stations  in  Ger- 
many, England,  and  this  country, — trained  experts,  under  the 
best  of  conditions,  are  subjecting  these  problems  to  the  test  of 
rigid  experiment.  The  results  are  sure  to  be  important  and  may 
be  revelational.  It  is  one  of  the  great  privileges  of  living  in  this 
age  that  we  may  witness  these  advances,  and  may  even  have 
part  in  them.  There  is  in  store  for  us  all,  who  are  students  of 
biological  science,  many  a  thrill  of  purest  delight  as  we  open  the 
pages  of  our  weekly  scientific  newspaper, — our  Nature  or  our 
Science, — and  find  the  first  announcement  of  discoveries  which 
will  later  illuminate  one  by  one  the  dark  problems  of  nature. 
Science  has  indeed  good  reason  for  her  distinctive  optimism. 


CHAPTER  XVII 

THE  REMARKABLE  IMPROVEMENT  MADE  IN  PLANTS 
BY  MAN,  AND  THE  WAY  HE  BRINGS  IT  ABOUT 

Plant  Breeding 


N  all  the  wide  range  of  relations  existing  between  plants 
and  mankind,  there  is  not  another  single  fact  which 
compares  in  importance  with  this, — that  plants  can  be 
altered  by  man  to  make  them  fit  better  his  needs  or  his 
fancy.  His  accomplishments  in  this  field,  indeed,  partake  of  the 
marvelous.  Everybody  knows  the  magnificent  exhibition  type 
of  Chrysanthemum,  with  its  superb  great  globular  head  of  snowy 
incurving  petals,  well-nigh  geometrical  in  the  perfection  of  its 
symmetry.  But  does  everyone  know  that  it  has  been  created  by 
man  out  of  two  daisy-like  plants  smaller  and  humbler  than  the 
commonest  weed  of  our  hayfields?  Likewise,  all  those  strongly 
individualistic  types  of  the  same  noble  flower, — the  prim  little 
pompon,  the  star-like  anemone,  the  stiffly-correct  reflex,  the 
shaggy  Japanese,  and  a  number  of  others,  a  few  of  which  are 
shown  clustered  together  upon  the  accompanying  plate  (fig- 
ure 174),  have  all  been  differentiated  from  the  same  unpromising 
beginning.  Again,  the  Bartlett  Pear,  huge  and  luscious,  has  been 
developed  within  three  hundred  years  from  a  small  stony  fruit 
attractive  to  no  one  except  vagabonds  and  omnivorous  small 
boys.  Indian  Corn  and  Wheat,  chief  of  the  food  plants  of  civilized 
man,  have  been  improved  so  far  from  the  simple  wild  grasses  with 
which  the  first  cultivators  had  to  begin,  that  Botanists  are  hardly 
yet  fully  agreed  as  to  what  those  wild  ancestors  were.  Oranges, 

426 


FIG.  174. — The  leading  types  of  Chrysanthemum,  all  developed  by  man  from  wild  an- 
cestors having  a  size  and  form  very  like  the  lowermost  single  flowers  of  the  picture. 


427 


428 


The  Living  Plant 


Bananas,  Pineapples  have  been  immensely  enlarged,  greatly  im- 
proved in  flavor,  and  actually  rendered  seedless.  Brilliantly  red 
foliage  plants,  including  some  trees,  have  been  derived  from 
green  forbears.  Invaluable  vegetables,  often  imposing  in  size, 
have  been  made  to  spring  from  insignificant  weeds,  as  in  case  of 
the  familiar  varieties  represented  in  the  accompanying  picture 


Fio.  175.— Representative  forms  of  Cabbage,  Kohl-rabi,  Cauliflower,  Brussels  Sprouts, 
and  Tree  Cabbage,  evolved  by  man  from  the  wild  shore  plant,  Brassica  oleracea,  of 
which  two  forms  are  shown  in  the  upper  left  hand  corner.  The  pictures  are  about 
one  twenty-fourth  of  the  natural  size.  (Redrawn  from  a  colored  wall-chart  by  Laurent 
and  Errera.) 

(figure  175).  Not  one  of  these  remarkable  productions,  or  any  of 
the  vast  number  of  which  they  are  typical,  would  exist  to-day, 
were  it  not  for  the  craft  and  the  patience  of  man.  It  is  now  our 
particular  task  to  inquire  in  exactly  what  way  this  indubitable 
miracle  has  been  wrought. 

The  methods  of  plant  improvement  are  few,  old,  simple,  and 


Improvements  Made  in  Plants  by  Man  429 

perfectly  known.  The  earliest  cultivators  made  use  of  them,  and 
the  most  scientific  of  horticultural  experts  have  no  others  to-day. 
For  convenience  of  study  we  may  consider  them  as  three  in 
number  and  distinct,  though  in  fact  they  are  interwoven  inex- 
tricably. They  are, — Selection  of  Variations;  Preservation  of 
Sports;  Crossing  and  Hybridization.  And  perhaps  the  reader  will 
here  add  in  his  mind  "and  also  Cultivation;"  but  he  would  be 
wrong.  Although  cultivation  can  produce  better  individuals,  it 
cannot  produce  of  itself  better  races,  for  the  two  are  not  the  same 
thing  at  all. 

1.  The  Selection  of  Variations. — The  reader  will  already  have 
noticed  the  very  close  connection  which  exists  between  Evolution, 
considered  in  the  preceding  chapter,  and  the  Improvement  of 
Plants  by  man,  or  Plant  Breeding,  our  immediate  subject, — a 
connection  which  explains  the  juxtaposition  of  the  two  chapters, 
and  is  not  badly  expressed  by  calling  Plant  Breeding  artificial 
evolution.  Moreover,  the  two  have  precisely  the  same  basis, — in 
Variation,  which  we  must  now  consider  rather  more  fully  than  was 
needful  before. 

If  all  the  plants  of  one  kind,  or  species,  were  born  exactly  like 
one  another,  as  crystals  are,  then,  so  far  as  we  can  see,  no  im- 
provement of  plants  by  man  would  be  possible.  But  plants  of 
the  same  species  are  not  born  alike  any  more  than  are  people  of 
the  same  race  or  even  the  same  blood.  In  a  field  of  Corn,  are  all 
the  plants  of  one  height,  or  have  they  the  same  number  of  grains 
to  the  ear?  Are  the  Elms  in  a  meadow  all  cast  in  the  same  mold 
of  grace?  Are  the  flowers  of  any  one  annual  precisely  alike  in 
their  hue?  There  is  an  experiment  which  my  students  try  every 
year,  with  a  result  that  is  always  surprising  to  them,  and  a  satis- 
faction to  me.  From  a  large  lot  of  seeds  of  the  same  variety  and 
crop,  they  select  a  definite  number  of  grains  just  as  similar  as 
possible  in  size,  form,  weight,  color  and  other  features;  these  they 
plant  at  exactly  equal  distances  apart,  at  the  same  depths  and  in 
the  same  positions,  in  a  box  of  evenly-mixed  earth,  which  is  then 


430  The  Living  Plant 

kept  watered,  warmed  and  lighted  uniformly  over  its  whole  sur- 
face. This  is  something  which  the  reader  can  readily  try  for 
himself,  and  I  commend  it  to  his  favorable  attention.  As  the 
plants  come  up,  the  differences  they  exhibit  in  rapidity  of  ger- 
mination, in  the  rate  of  subsequent  growth,  and  in  every  detail  of 
their  structure,  are  most  striking,  as  the  accompanying  pictures, 
traced  from  photographs,  to  some  extent  illustrate  (figure  176). 
In  their  main  features,  it  is  true, — those  by  which  we  distinguish 


Fiu.  17(5. — Young  seedlings  of  String  Bean  and  of  Corn,  grown  from  seeds  as  nearly  alike 
in  all  visible  features  as  possible,  and  planted  exactly  alike.  Traced  from  a  photograph. 

them, — plants  of  the  same  species  are  alike,  but  in  their  details 
they  are  always  different,  and  often  conspicuously  so.  Plants  of 
the  same  kind  are,  as  it  were,  alike  in  general  but  different  in 
particular.  The  matter  is  sometimes  expressed  in  this  way,  that 
no  living  being  is  just  like  any  other  living  being, — a  statement 
impossible  of  logical  proof,  but  shown  by  experience  to  be  true 
for  all  practical  purposes. 

Turning  now  to  a  more  exact  examination  of  these  differences, 
or  variations,  we  find  that  they  arise  from  diverse  causes.  A 
part  of  them  are  of  purely  mechanical  origin,  being  forcibly  im- 
posed upon  some  individuals,  but  not  others,  by  overcrowding, 
attacks  of  enemies,  or  lack  of  suitable  nourishment.  Thus,  in- 
sufficiency of  water  causes  the  dwarfing  of  plants  simply  because 
they  have  not  enough  water  with  which  to  grow  big;  and  man  can 
make  individual  plants  short-stemmed  in  this  way  if  he  wants  to. 
Another  part  of  the  variations  arise  from  the  remarkable  power 


Improvements  Made  in  Plants  by  Man  431 

which  individuals  have  of  adjusting  their  parts  to  peculiarities  in 
the  distribution  of  light,  moisture,  minerals  or  other  essential 
conditions  of  their  immediate  surroundings, — a  power  which  we 
have  studied  with  some  care  in  the  chapter  on  Irritability.  An 
example  thereof  is  the  greater  lengthening  of  stems  (which  are 
"drawn,"  in  the  language  of  gardeners)  when  exposed  to  insuffi- 
cient light,  and  of  which  the  very  long  shoots  formed  by  potatoes  in 
cellars  are  an  extreme  example.  Upon  these  stems  the  deficiency 
of  light  acts  as  a  stimulus,  making  them  lengthen  out  as  if  in  the 
effort  to  carry  their  leaves  into  full  brightness;  and  man  can  make 
plants  long-stemmed  in  this  way  if  he  wants  to.  But  when  all  of 
the  differences  due  to  mechanical  causes  or  to  irritability  are 
eliminated,  as  can  largely  be  done  by  careful  experiment,  there 
remains  always  a  great  residue  of  differences  for  which  there  is  no 
conceivable  origin  except  that  they  are  innate  or  inborn  in  the 
plants  themselves.  Thus,  when  all  external  conditions  of  water- 
supply,  light,  &c.,  are  carefully  made  the  same  for  all  the  plants 
under  our  experiment  above  described,  the  stems  of  the  plants 
nevertheless  differ  in  length,  some  being  shorter  and  some  longer. 
And  man  can  also  obtain  long-stemmed  plants  in  this  way  if  he 
wants  to.  It  is  thus  plain  that  the  differences  between  individual 
plants  of  the  same  species  arise  in  at  least  three  different  ways, 
which  we  may  summarize  in  an  order  the  reverse  of  that  of  their 
treatment  above,  by  saying, — that  in  some  of  their  features  plants 
are  born  different,  others  of  their  differences  are  achieved,  while 
some  of  their  differences  are  thrust  upon  them. 

Of  the  three  kinds  of  differences,  the  inborn  variations  are  the 
only  ones  important  in  the  improvement  of  plants,  as  in  natural 
evolution,  and  for  the  same  reason,  that  they  are  the  only  kind 
which  can  be  transmitted  to  descendants.  Although  man  is  able 
by  regulation  of  the  water  or  light  supply  to  make  individual 
plants  short-stemmed  or  long-stemmed,  he  cannot  by  this  means 
make  a  short-stemmed  or  long-stemmed  race  which  will  reproduce 
itself  generation  after  generation.  The  only  known  way  in  which 


432  The  Living  Plant 

he  can  obtain  such  a  race  is  by  watching  for  plants  which  naturally 
exhibit  an  inborn  short-stemmed  or  long-stemmed  variation,  re- 
spectively, selecting  them  out  and  propagating  them ;  the  short-  or 
long-stemmed  character  will  appear  in  their  descendants,  and  by 
consistent  repetition  of  selection  of  the  same  variation  for  some 
generations,  a  race  capable  of  perpetuating  the  short-  or  long- 
stemmed  character  can  be  obtained.  The  fact,  then,  that  innate 
variations  are  hereditary  is  the  most  important  fact  about  them 
from  the  point  of  view  of  plant  improvement. 

There  is  one  other  point  about  the  heredibility  of  variations 
which  we  must  note  at  this  place.  It  was  Darwin's  view  that 
variations  rise  and  fall,  or  flash,  as  it  were,  across  several  genera- 
tions, and,  unless  preserved  by  selection,  sooner  or  later  die  out. 
But  modern  studies  are  showing  that  variations  appear  linked 
several  together  in  those  mutants,  biotypes,  or  elementary  species 
which  we  have  already  considered  in  the  preceding  chapter, 
while,  once  in  existence,  they  persist  indefinitely.  What  then, 
on  the  newer  view,  is  the  gardener  doing  when  selecting  vari- 
ations for  the  improvement  of  plants?  Simply  this, — he  is 
isolating  the  desirable  biotypes  from  among  the  less  desirable 
kinds  making  up  the  great  mixture  of  which  any  crop  consists. 
Thus  a  field  of  Corn  or  Wheat  consists  of  a  great  number  of  bio- 
types, and  hybrids  thereof,  from  which  the  best  kinds  can  be 
selected  and  propagated.  But,  on  the  newer  view,  once  the  best 
biotypes  are  isolated,  no  further  improvement  is  possible,  while 
the  selection  of  variations  should  permit  an  indefinite,  or  at  least 
much  larger  improvement.  Experience  is  certainly  showing  the 
truth  of  the  modern  view  in  many  cases,  though  the  accumulation 
of  single  small  variations  seems  equally  clear  in  other  instances. 

A  second  great  fact  about  variation  is  this, — it  is  spontaneous, 
which  means  that  it  appears  without  any  determinable  reference 
to  any  features  of  the  surroundings.  But  while  the  surroundings: 
do  not  in  any  known  way  determine  the  nature  of  variations,  they 
certainly  do  promote  them,  both  in  number  and  intensity,  as 


Improvements  Made  in  Plants  by  Man  433 

shown  by  the  fact  that  variations  become  more  active  when  the 
external  conditions  are  changed.  This  happens  when  plants  are 
taken  to  new  countries;  or  are  brought  out  of  forest  or  field 
into  garden  or  greenhouse;  or  are  subjected  to  high  cultivation, 
through  which  are  provided  better  conditions  for  nourishment  and 
comparative  freedom  from  natural  enemies;  or  are  given  different 
soils,  fertilizers  and  exposures;  or  are  crossed  in  reproduction, — 
a  matter  which  we  shall  consider  more  fully  in  a  moment.  There 
are,  of  course,  limits  to  the  change  of  conditions  that  plants  can 
endure,  but  within  those  limits  all  changes  in  external  conditions 
are  followed  by  more  rapid,  diverse,  and  extreme  variation.  Va- 
riation in  organisms  may  be  symbolized  by  the  gentle  trembling  of 
the  surface  of  water  held  in  a  full  dish  at  arm's  length;  if  the 
hands  are  deliberately  shaken  a  little,  the  trembling  increases, 
though  the  shaking  must  be  kept  within  limits,  else  the  water  is 
spilled  from  the  dish.  The  cultivators  of  plants,  realizing  well 
that  variation  is  the  basis  of  the  possibility  of  improving  plants, 
and  observing  that  change  in  conditions  promotes  it,  have  from 
the  earliest  times  made  use  of  these  methods  for  rendering  more 
variable  those  forms  which  they  wish  to  improve,  but  which 
naturally  exhibit  little  variation.  This  is  precisely  what  they 
mean  by  their  expression  "  break  the  type." 

A  third  vital  fact  about  variation  is  this :  it  is  fortuitous,  which 
means  that  it  takes  place  in  any  possible  part,  feature,  or  direc- 
tion, indifferently,  according  to  chance,  and  shows  no  tendency 
to  follow  any  particular  lines,  excepting  in  so  far  as  structural 
conditions  may  make  it  easier  to  vary  one  way  than  another. 
Stems  do  not  vary  towards  shortness  alone  nor  longness  alone, 
but  towards  shortness,  longness,  thickness,  thinness,  roundness, 
angularity,  flexibility,  stiffness,  and  any  other  peculiarities  which 
the  construction  of  stems  makes  it  possible  for  them  to  exhibit. 
Moreover,  these  variations  insist,  so  to  speak,  upon  originating 
and  directing  themselves,  and  all  the  ingenuity  of  man  has  not 
yet  enabled  him  either  to  originate  or  to  determine  the  direction 


434  The  Living  Plant 

of  any  desired  variation  in  any  given  plant.  Therefore  his  only 
resource  is  to  wait  until  the  desired  variation  appears,  which  will 
be  the  sooner  the  larger  the  number  of  plants  that  he  deals  with, 
and  the  more  actively  he  employs  the  devices  for  "  breaking  the 
type."  Under  these  conditions,  it  is  only  a  question  of  time  when 
any  desired  variation  that  is  mechanically,  physically,  or  chem- 
ically possible  will  appear,  after  which  it  can  be  selected  and 
intensified  by  the  methods  already  described. 

Let  us  now  illustrate,  by  a  suppositional  case,  the  way  in  which 
man  makes  use  of  variation  in  improving  some  particular  kind  of 
plant.  Let  us  suppose  that  out  of  a  race  of  white-flowered  plants, 
the  breeder  desires  to  develop  a  red-flowered  variety.  He  knows 
it  is  useless  to  try  to  turn  the  flowers  red  directly,  by  chemicals  in 
the  soil,  regulation  of  the  light,  or  anything  of  that  kind,  for  al- 
though white  flowers  might  conceivably  be  made  red  by  such 
methods,  the  redness  would  not  be  transmitted,  and  the  next 
generation  would  be  just  as  white  as  ever.  He  knows  that  his 
only  chance  of  success  lies  in  the  spontaneous  appearance  of  a 
strain  of  red  color,  and  accordingly  he  grows  just  as  many  plants 
as  he  can  possibly  find  space  for,  giving  them  diverse  conditions 
of  soil,  fertilizers,  situation  and  cultivation,  in  an  effort  to  break 
the  white  type.  Inevitably,  sooner  or  later,  unless,  indeed,  as 
sometimes  happens,  there  is  some  chemical  obstacle  in  the  con- 
stitution of  the  plant,  some  redness  will  appear,  faintly  perhaps 
but  unmistakably,  in  some  of  the  white  blossoms.  He  then 
isolates  those  plants,  remorselessly  destroying  all  the  remainder, 
and  breeds  them  together  if  possible,  though  this  is  by  no  means 
indispensable.  From  the  seeds  of  the  selected  plants  he  raises  as 
many  as  he  can,  and  amongst  their  flowers  though  some  revert 
back  to  whiteness,  the  majority  are  likely  to  show  the  red  strain  of 
the  parents,  while  a  few  (though  perhaps  not  for  another  genera- 
tion or  two)  will  exhibit  a  still  redder  strain.  The  latter,  of  course, 
are  then  selected,  and  bred  together,  and  their  seeds  are  sown  as 
before.  In  the  resulting  generation  will  appear  fewer  white 


Improvements  Made  in  Plants  by  Man  435 

flowers  than  before,  a  larger  proportion  of  red  like  the  parents, 
and  perhaps  a  still  redder  strain,  though  again  this  may  not 
appear  for  a  number  of  generations.  Thus,  gradually,  generation 
after  generation,  the  quality  of  redness  becomes  extended  and 
intensified,  while  whiteness  diminishes  to  final  disappearance;  so 
that  finally  a  permanently  red-flowering  race  has  been  secured. 
Whether,  however,  this  process  depends  chiefly  upon  the  selection 
and  accumulation  of  red  variations  in  the  Darwinian  sense,  or 
upon  the  isolation  of  successively-appearing  new  biotypes,  I  do 
not  know,  but  expect  the  near  future  to  decide. 

In  any  case,  the  improvement  of  plants  by  the  selection  of 
variations  is  a  slow  process,  and  it  is  fortunate  that  a  far  more 
rapid,  even  though  rather  spasmodic  method  exists,  viz.,  the 
preservation  of  sports. 

2.  The  Preservation  of  Sports. — The  most  of  my  readers,  I  fancy, 
know  something  about  sports  among  plants.  The  most  typical 
ones  originate  like  this.  On  some  ordinary  plant  a  single  bud, 
differing  visibly  in  no  wise  from  its  neighbors,  grows  out  to  a 
branch  which  bears  leaves,  fruits  or  flowers  conspicuously  dif- 
ferent from  all  others  on  that  plant.  About  five  years  ago,  in 
a  greenhouse  where  I  teach,  a  certain  Pompon  Chrysanthemum 
bearing  pretty  pink  flowers  put  forth  a  single  branch  on  which 
all  of  the  flowers  were  entirely  different,  being  a  striking  bronze 
brown.  For  that  season  the  plant  was  a  wonder  to  visitors,  who 
delighted  to  represent  that  they  could  hardly  believe  their  eye- 
sight; a  pride  to  the  students,  who  accepted  its  appearance  as  a 
delicate  compliment  to  themselves;  and  a  treasure  to  me,  who 
made  the  best  of  this  unusual  educational  opportunity.  From 
both  the  sporting  and  the  ordinary  branches  we  took  cuttings, 
and  from  these  the  next  season  we  grew  two  plants  of  the  re- 
spective colors,  which  we  have  propagated  continuously  to  this 
day.  On  an  ordinary  green  beech  tree  in  Scotland  somewhat 
less  than  a  century  ago,  a  single  one  of  the  innumerable  buds 
grew  into  a  branch  on  which  every  leaf  was  dark  red.  That 


436  The  Living  Plant 

branch  was  propagated  by  grafting  and  these  plants  by  graft- 
ing again;  and  such  was  the  origin  of  all  those  favorite  lawn 
trees  which  we  call  copper  beeches.  On  an  ordinary  orange 
tree,  not  so  many  years  ago,  a  single  bud  produced  a  branch 
which  bore  only  seedless  fruit,  the  seedlessness  being  correlated 
with  the  presence  of  a  tiny  accessory  orange  embedded  almost 
wholly  in  the  larger;  that  branch  was  grafted,  as  were  the  result- 
ing branches;  and  this  is  the  origin  of  the  thousands  of  trees  now 
bearing  the  navel  orange.  Nectarines  are  bud  sports  from  peach 
trees,  and  most  of  our  finest  varieties  of  apples,  of  pears,  and  of 
many  other  fruits  have  originated  in  just  such  a  manner.  Haw- 
thorns, Azaleas,  Pelargoniums  (" Geraniums"))  Roses,  Carna- 
tions, and  many  other  plants  sport  greatly  in  the  color  of  their 
flowers;  and  many  of  our  choicest  varieties  of  these  charming 
plants,  including  double-flowered  forms,  came  thus  into  existence, 
as  did  many  of  our  cut-leaved  trees,  variegated  plants,  and 
crested  oddities.  Sometimes  the  sports  take  curious  directions, 
as  in  the  case  of  branches  which  bear  leaves  that  unfold  in  the 
spring  several  days  in  advance  of  any  others  on  the  tree.  I  know 
a  tulip  bulb  which,  year  after  year,  produces  flowers  highly 
doubled  and  accompanied  by  a  colored  peduncular  bract.  This 
latter  case  is  interesting  as  marking  a  transition  over  to  those 
peculiar  structures  called  monstrosities,  which  are  largely,  though 
not  always,  sports.  Indeed  a  monstrosity  is  usually  but  a  sport 
which  strikes  us  as  somewhat  abnormal  or  "  queer, "  such  for 
example  as  green  roses,  crested  plants,  and  other  " freaks,"  all  of 
which  can  be  propagated  regularly  by  cuttings.  And  other 
sports  are  known  in  great  number  as  recorded  in  the  books  devoted 
to  horticulture.  But  as  to  this,  one  must  not  forget  that  only 
those  sports  which  appeal  to  man  as  useful,  attractive,  or  curious, 
are  likely  to  receive  mention  in  such  books,  while  innumerable 
others,  which  make  their  appearance  but  have  no  interest  to  man, 
are  left  in  oblivion. 

Concerning  the  causes  of  bud  sports  we  know  as  little  as  we  do 


Improvements  Made  in  Plants  by  Man  437 

concerning  the  causes  of  variation,  the  two,  indeed,  being  doubt- 
less identical  in  nature.  Like  variations,  bud  sports  are  spon- 
taneous and  fortuitous  and  can  be  rendered  more  frequent  by 
cultivation;  and  they  are  hereditary  through  the  new  buds  they 
produce,  though  never  by  seeds.  Everybody  knows  that  a 
Bartlett  Pear  or  a  Baldwin  Apple  can  be  propagated  by  grafting, 
but  not  by  the  seeds,  which  do  not  produce  those  fruits,  but  just 
plain  ordinary  mongrel  pears  and  apples.  Were  it  not  for  the 
fact  that  most  of  the  plants  which  produce  bud  sports  can  be 
propagated  by  slips,  or  cuttings,  or  else  by  grafting  (which  is 
merely  a  process  of  giving  ready  formed  roots  to  slips  unable  to 
make  roots  for  themselves),  it  would  not  be  possible  to  preserve 
bud  sports,  and  they  would  perish  with  the  plants  which  produce 
them.  But  those  methods  of  propagation  do  permit  man  to 
preserve  them,  to  his  very  great  advantage. 

Sports  from  buds,  however,  are  not  the  only  kind,  for  seed 
sports  also  occur,  though  upon  the  whole  they  are  less  conspicuous 
and  important  than  bud  sports.  Among  brilliantly  red  cardinal 
flowers,  some  plants  occasionally  occur  with  pure  white  flowers; 
when  seeds  from  the  white  flowers  are  planted,  they  produce 
white-flowering  plants.  The  white  cardinal  flowers  are  typical 
seed  sports,  and  the  fact  that  they  propagate  by  seed  is  typ- 
ical. Some  cut-leaved  trees  (e.  g.  Wier's  cut-leaf  Maple),  and 
some  fine  varieties  of  fruits  have  also  originated  as  seed  sports. 
Whether  these  trees  come  true  to  seed  I  do  not  know,  and  the 
matter  is  not  practically  important,  since  they  can  be  propagated 
far  more  speedily  and  easily  by  grafting.  It  is  obvious  that  seed 
sports  which  propagate  their  characters  through  seeds  differ  in 
no  essential  particular,  except  perhaps  that  of  degree,  from  the 
mutants,  or  biotypes,  described  in  the  chapter  on  Evolution. 

3.  Crossing  and  Hybridization. — The  reader  will  recall  that  seed- 
formation  must  be  preceded  by  fertilization,  which  in  turn  re- 
quires that  the  pollen-grain  containing  a  male  cell  shall  be  trans- 
ferred from  anthers  where  it  is  made,  to  a  stigma  giving  access 


438  The  Living  Plant 

to  the  female  cell  in  the  ovule.  Now  in  nature  this  transfer  is 
effected  by  the  agency  of  wind,  water,  or  insects,  but  in  culti- 
vation man  can  transfer  the  pollen  himself  if  he  pleases,  and 
can  thus  to  some  extent  control  the  parentage  of  the  new  in- 
dividuals formed  in  the  ovules.  And  these  are  the  various  com- 
binations he  can  make : — 

(1)  He  can  pollinate  a  given  stigma  by  pollen  from  the  same 
flower.    This  is  called  close  pollination  in  Botany  and  in-breeding 
in  Horticulture.     With  many  plants,  perhaps  the  majority,  no 
result  follows,  for  the  seed  does  not  set,  the  ovule  being  sterile  to 
the  pollen  of  the  same  flower;  while  in  many  kinds  of  flowers  such 
pollination  is  impossible,  because  the  pollen  and  ovules  in  each 
single  flower  ripen  at  different  tunes,  or  because  of  other  im- 
pediments.   It  is  thus  obvious  that  nature  takes  trouble,  so  to 
speak,  to  prevent  such  in-breeding;  and  the  implication  that  such 
breeding  is  in  general  not  advantageous  is  confirmed  by  such 
evidence  from  experiment  as  we  possess,  which  shows  that  the 
offspring  of  close  in-breeding  are  generally  inferior  in  variability 
if  not  in  vigor  to  those  more  widely  bred.    But  in  the  very  fact 
that  in-breeding  does  not  favor  variability  is  found  its  chief  horti- 
cultural importance,  for  it  can  be  used  to  keep  a  race  true  to  its 
type  when  that  is  desirable.     In  practice,  however,  such  use  is 
very  limited  because  the  same  result  can  be  attained  much  more 
easily  in  most  plants  by  propagation  through  cuttings  or  grafting, 
and  by  the  systematic  weeding  out  of  all  plants  (horticulturally 
called  "rogues")  which  show  individual  variations. 

(2)  He  can  pollinate  a  given  stigma  by  pollen  from  another 
flower  on  the  same  plant.    This  also  is  in-breeding,  and  experi- 
ments have  shown  that  the  results  are  little  if  any  better  than  in 
the  case  of  the  first  method. 

(3)  He  can  pollinate  a  given  stigma  by  pollen  from  a  different 
plant  of  the  same  kind.    This  is  called  cross  pollination  in  Botany, 
and  crossing  in  Horticulture,  and  is  that  to  which  most  of  the 
cross-pollinating  mechanisms  and  methods  in  flowers  are  adapted. 


Improvements  Made  in  Plants  by  Man  439 

It  is  both  known  from  experience,  and  also  has  been  shown  by 
experiment,  that  crossing  yields  more  vigorous,  abundant,  and 
variable  offspring  than  in-breeding,  in  which  fact  lies  the  reason, 
doubtless,  why  nature  promotes  it.  In  crossing,  not  only  is  va- 
riability promoted  by  the  introduction  of  the  peculiarities  of  two 
lines  of  ancestry,  but  the  very  commingling  of  the  two  strains  of 
protoplasm  seems  to  favor  the  appearance  of  wholly  new  varia- 
tions, somewhat  after  the  analogy  of  these  liquids  in  chemistry, 
which  are  perfectly  clear  by  themselves  but  turbid  when  com- 
mingled. It  appears  also  to  be  a  fact  that  the  offspring  are  more 
vigorous,  prolific,  and  variable  yet,  if  the  two  plants  between 
which  the  cross  is  made  are  not  raised  side  by  side  under  the 
same  conditions,  but  apart  and  under  somewhat  different  condi- 
tions. Cross  pollination  between  plants  thus  grown  is  something 
which  nature  cannot  provide  for,  but  man  can,  and  sometimes 
does,  as  when  he  plants  seeds  in  alternate  rows  treated  somewhat 
differently  in  cultivation.  By  this  method  man  can  intensify 
variation,  and  thus  provide  a  wider  and  better  basis  for  selection. 
(4)  He  can  pollinate  a  given  stigma  by  pollen  from  a  plant  of 
another  variety  of  the  same  species.  This  is  called  hybridization 
in  both  Botany  and  Horticulture,  though  by  some  it  is  also  desig- 
nated crossing.  Occasionally  no  result  follows,  but  usually  seed 
sets  and  will  grow  into  new  plants  which  breed  freely  together. 
The  first  generation  of  such  hybrid  progeny  exhibit  characters  de- 
rived from  both  of  the  original  parents,  and  are  likely  to  be  more 
vigorous  than  those  parents;  but  (and  this  will  be  news  to  many 
of  my  readers),  these  characters  are  not  combined  in  the  same 
way  in  all  of  the  descendants  of  those  hybrids,  though  the  ways 
are  very  definite.  This  very  important  matter,  which  involves  a 
notable  natural  law  discovered  by  Mendel  and  known  by  his  name, 
has  already  been  considered  in  the  chapter  on  Reproduction,  and 
it  will  suffice  to  recall  here  that  the  characters  of  the  original 
parents  are  inherited  by  the  descendants  of  hybrids  as  definite 
entities  in  definite  mathematical  proportions.  This  fundamental 


440  The  Living  Plant 

fact  has  a  practical  consequence  of  the  first  importance,  since  it 
is  thus  made  possible  for  a  breeder  so  to  breed  the  hybrids  to- 
gether as,  on  the  one  hand,  to  eliminate  utterly  out  of  the  hybrid 
race  any  given  undesirable  quality  inherited  from  either  of  the 
original  parents,  and,  on  the  other,  to  combine  and  fix  perma- 
nently in  the  race  any  two  given  desirable  qualities  originally 
occurring  in  separate  parent  races.  Thus,  it  has  been  possible 
to  produce  hybrid  races  of  wheat  in  which  the  superior  flour- 
producing  quality  of  one  variety  has  been  united  with  the  superior 
frost-resisting  quality  of  another,  the  inferior  frost-resisting 
quality  of  the  former,  and  the  inferior  flour-producing  qualities 
of  the  latter  having  been  eliminated  permanently  out  of  the 
hybrid  race.  Moreover,  it  is  possible,  theoretically  at  least,  thus 
to  combine  in  one  race  any  number  of  good  qualities  from  any 
number  of  different  varieties  of  a  species,  though  in  practice,  as 
we  shall  see  in  a  moment,  the  matter  is  attended  with  immense 
practical  difficulties.  It  is,  however,  in  this  possibility  of  com- 
bining in  one  race  the  desirable  qualities  from  different  races 
while  eliminating  the  opposite  qualities,  that  the  highest  utility 
of  hybridization  in  connection  with  the  improvement  of  plants 
consists. 

(5)  He  can  pollinate  a  given  stigma  by  pollen  from  a  plant  of 
another,  but  allied,  species.  This  also  is  called  hybridization,  in 
both  Botany  and  Horticulture.  In  the  vast  majority  of  such 
pollinations  no  result  follows,  but  in  the  few  cases  where  seed  is 
formed,  the  derived  specific  hybrids,  like  the  varietal  hybrids 
just  considered,  exhibit  characters  derived  from  both  parents,  as 
also  new  characters  not  traceable  to  either.  Like  the  first  genera- 
tion of  varietal  hybrids,  also,  they  are  often  larger  and  more 
vigorous  than  either  parent;  but  on  the  other  hand  they  are  al- 
most invariably  defective  in  reproductive  power,  and  can  hardly 
ever  reproduce  by  seeds.  A  famous  example  of  a  species  hybrid  is 
Lilium  Parkmanni,  a  magnificent  Lily  even  finer  than  either  of  its 
superb  parents,  but  it  is  rarely  seen  in  gardens  because  it  does  not 


Improvements  Made  in  Plants  by  Man  441 

reproduce  at  all  by  seeds  and  only  badly  by  bulbs.  Such  of  these 
species  hybrids,  however,  as  can  be  reproduced  by  cuttings  or 
grafting  can  be  preserved  indefinitely,  and  this  is  the  case  with 
hybrid  trees,  because  the  seedling  can  be  grafted  into  either  of  the 
parent  trees,  or  into  some  allied  kind,  and  thereafter  can  be  multi- 
plied with  rapidity  and  certainty.  It  is  obvious  from  these  con- 
siderations that  specific  hybrids  can  only  rarely  be  made  the 
foundation  of  a  race,  and  equally  plain  that  they  can  play  no 
appreciable  part  in  the  natural  evolution  of  plants. 

(6)  He  can  pollinate  a  given  stigma  by  pollen  from  a  plant  of 
another  but  allied  genus.  Though  some  such  generic  hybrids 
have  been  made  as  a  matter  of  scientific  experiment,  this  has 
only  been  possible  with  genera  extremely  closely  related,  and 
moreover  the  hybrids  are  unstable  and  of  no  horticultural  im- 
portance. Nor  have  any  attempts  at  hybridization  over  wider 
limits  ever  succeeded;  and  the  occasional  newspaper  accounts  of 
crosses  between  members  of  different  plant  families  are  lies,  when 
they  are  not  obvious  jokes. 

It  is  evident  from  this  discussion  that  plant-breeders  make  use 
of  in-breeding,  crossing,  and  hybridization  for  various  purposes  in 
accordance  with  the  results  which  they  wish  to  attain.  By  suit- 
able combinations  it  is  possible  to  keep  races  close  to  their  type 
and  thus  preserve  desirable  characteristics ;  to  break  the  type  and 
thus  provide  a  basis  for  the  development  of  new  characteristics 
through  selection;  to  eliminate  undesirable  features  out  of  a  race; 
to  combine  the  desirable  features  of  two  or  more  races  into  one; 
and  in  general  to  promote  vigor  and  productivity.  I  think  it  will 
now  be  evident  why  crossing  and  hybridization  are  so  prominent 
in  plant  improvement. 

It  will  interest  the  reader,  at  this  point,  to  learn  in  what  way 
crossing  and  hybridization  are  effected  in  practice.  It  is  no 
trouble  at  all  to  transfer  the  pollen  from  any  ripe  stamens  to  any 
ripe  stigma.  It  is  only  necessary  to  pick  the  fine  pollen  dust  from 
the  opened  anthers  by  some  dry  utensil  to  which  it  will  cling  (for 


442  The  Living  Plant 

which  purpose  a  common  camelshair  paint  brush  is  admirable 
and  usually  employed),  and  then  press  it  against  the  desired 
stigma,  which  is  sticky  and  to  which  therefore  the  pollen  adheres. 
The  difficulty  in  the  process  comes  from  the  fact  that  undesired 
pollen  may  also  reach  the  stigma,  and  effect  a  wrong  fertilization. 
It  is  therefore  necessary  to  prevent  access  of  any  pollen  except 
that  deliberately  placed  upon  the  stigma  by  the  experimenter. 
Close  pollination  is  prevented,  in  those  plants  which  allow  it, 
by  snipping  off  the  stamens  before  the  anthers  are  ripe;  while 
undesired  cross  pollination  is  prevented  by  use  of  a  gauze,  or  thin 
paper  bag  kept  closely  tied  over  the  flower  except  at  the  moment 
when  the  desired  pollen  is  applied  to  the  ripe  stigma.  The  sight  of 
such  bagged  plants  must  be  familiar  to  all  those  who  have  visited 
agricultural  experiment  stations,  and  they  are  shown  in  figure  173. 

The  reader  will  no  doubt  be  surprised  that  in  this  discussion  I 
have  laid  no  more  stress  upon  cultivation,  which  surely,  he  will 
say,  does  much  improve  plants.  Cultivation  consists  in  giving 
to  plants  such  conditions  of  space,  nourishment,  and  freedom 
from  enemies  as  will  permit  them  to  develop  to  the  highest  degree 
that  their  internal  capacities  allow.  It  produces,  therefore,  better 
individuals  and  crops.  But  it  does  not  produce  better  races, 
because,  as  we  know,  the  good  effects  of  cultivation  are  chiefly 
irritable  responses  whose  results  are  never  transmitted  to  the  next 
generation.  Indirectly,  however,  cultivation  does  help  in  racial 
improvement,  for  on  the  one  hand  all  offspring  are  benefited  by 
greater  physical  health  in  their  parents,  and,  on  the  other,  with 
greater  physical  vigor  goes  greater  variability  and  tendency  to 
production  of  sports, — those  foundations  of  the  improvement  of 
races.  Just  as  the  best  nourished  animals  play  more  vigorously 
than  the  ill-nourished,  so  the  best  cultivated  plants  vary  and  sport 
the  most  actively, — from  very  excess  of  physical  vigor,  no  doubt. 

In  my  discussion  of  this  subject  thus  far,  I  have  made  it  an 
aim,  as  elsewhere  through  this  book,  to  exhibit  the  theory,  so  to 
speak,  of  the  subject.  For  this  purpose  I  have  had  to  separate 


Improvements  Made  in  Plants  by  Man  443 

out  the  constituent  methods  and  discuss  each  by  itself.  But 
thereby  I  have  given  the  matter  an  aspect  of  simplicity  which 
it  is  far  from  deserving,  for  not  only  are  the  methods  inextricably 
interconnected,  but  practical  difficulties  of  innumerable  sorts 
interpose  large  obstacles  to  their  successful  operation.  Thus, 
I  have  spoken  of  plant  breeding  as  it  would  be  conducted  when  a 
matter  of  deliberate  intention  on  the  part  of  a  worker  with  a 
definite  idea  in  his  mind.  This,  however,  it  rarely  is  except  in  the 
case  of  modern  scientific  plant-breeders,  wealthy  amateurs,  or  a 
few  far-sighted  commercial  dealers  in  horticultural  novelties,  of 
whom  the  most  conspicuous  by  far  is  Luther  Burbank,  well  known 
of  late  to  the  readers  of  periodical  literature.  As  a  matter  of  fact, 
most  plant  improvement  has  been  made  on  the  spur  of  the  mo- 
ment, by  the  selection  of  something  which  happened  to  please 
the  fancy,  or  appeal  to  the  sense  of  profit,  of  gardener  or  farmer, 
who  of  course  has  always  sought  to  propagate  from  the  plants  he 
considers  his  "best. "  But  the  art  of  horticulture  is  long,  and  the 
life  of  man  is  short,  and  fancies  change,  and  things  that  are  profit- 
able vary;  wherefore  improvement  has  been  spasmodic,  and 
along  most  devious  courses.  Nor  are  horticultural  productions 
wholly  stable  when  once  secured,  for  varieties,  even  when  true  to 
their  good  character  for  a  time,  tend  strongly  to  revert  or  merge 
off  or  "wear  out"  to  less  desirable  kinds,  though  there  is  perhaps 
a  difference  between  mutations  which  are  permanently  stable,  and 
the  results  of  the  selection  of  small  variations,  which  are  unstable. 
Furthermore,  hybridization,  especially  for  the  combining  of 
features  from  different  races  into  one,  is  by  no  means  so  simple  as 
its  theory  implies,  but  a  process  distinguished  by  innumerable 
failures,  and  requiring  a  persistence  and  skill  that  few  breeders 
command.  The  potentialities  of  improvement,  indeed,  have  a 
vast  burden  of  practical  troubles  to  carry;  and  it  is  this  which 
makes  its  progress  so  halting  and  laborious. 

It  is  evident  that  in  his  improvement  of  plants,  man  never 
creates,  except  by  a  figure  of  speech,  but  only  directs.    He  cannot 


444  The  Living  Plant 

compel  plants  to  go  as  he  wishes,  but  he  can  lead  them  in  any 
direction  they  are  capable  of  going.  The  forces  of  improvement 
lie  deep  inside  of  the  plants  themselves,  seething  and  smoldering 
ready  for  an  outbreak;  all  that  man  can  do  is  to  suppress  or  bank 
them  in  places  where  they  are  doing  no  good,  and  give  them  free 
vent  where  they  can  produce  beneficent  results.  The  method  of 
effecting  desired  results  through  the  guidance  of  internal  forces  is 
not,  however,  confined  to  plant  breeding,  but  is  that  by  which  all 
great  organizers  of  large  enterprises,  and  all  great  leaders  of  men, 
effect  their  successes.  By  this  method  they  succeed  when  those 
who  try  to  force  the  improvement  from  without  meet  only  with 
failure. 


CHAPTER  XVIII 

THE  PRINCIPAL  GROUPS  INTO  WHICH  PLANTS  NAT- 
URALLY FALL,  WHETHER  BY  RELATIONSHIP  OR 
HABIT 

Classification 

NE  does  not  go  far  with  the  study  of  plants  before  he 
perceives  that  they  fall  into  groups,  and  groups  within 
groups,  according  to  the  degrees  of  their  likenesses  and 
differences.  Some  kinds  are  so  closely  alike  that 
botanical  experts  dispute  as  to  whether  they  really  are  different 
or  merely  two  forms  of  the  same,  while  others  are  so  very  unlike 
that  they  offer  not  the  least  point  of  resemblance;  and  there  is 
every  gradation  between.  The  arrangements  of  plants  in  their 
groups,  and  of  these  in  relation  to  one  another,  is  Classification, 
which  we  must  now  proceed  to  consider  in  so  far  as  it  has  connec- 
tion with  the  particular  theme  of  this  book.  And  we  naturally 
begin  with  the  groups  which  are  largest  and  best  defined,  of  which 
there  are  five, — Algae,  Fungi,  Moss-Plants,  Fern-Plants,  and 
Seed-Plants. 

The  Algae. — These  are  the  distinctive  plants  of  the  waters, 
comprising  especially  the  Seaweeds,  but  also  many  kinds  that 
dwell  in  rivers  and  lakes,  and  a  few  that  live  out  in  the  air.  In 
size  they  range  widely,  from  kinds  too  small  for  the  eye  to  detect 
up  to  the  great  Macrocystis  of  the  Pacific,  whose  thousand  feet 
(pretty  nearly)  of  length  surpasses  anything  that  land  plants  can 
offer.  In  shape  they  are  bewilderingly  multifarious, — spheres, 
cylinders,  hairs,  plates,  tufts,  fronds,  and  even  leafy  stems,  which 

445 


446  The  Living  Plant 

latter  bear  a  striking,  albeit  superficial,  resemblance  to  those 
parts  in  the  higher  land  plants.  Whatever  their  shapes,  they 
exhibit  a  wide  prevalence  of  minuteness,  thinness,  or  fine  division 
of  structure,  these  features  being  correlated  with  the  comparative 
scarcity  of  the  indispensable  gases,  which  they,  like  the  fishes, 
must  take  from  solution  in  the  water.  In  color  they  are  typically 
green  from  the  presence  of  chlorophyll,  by  aid  of  which  they  make 
their  own  food  precisely  in  the  manner  of  the  familiar  green  land 
plants;  but  in  a  good  many  kinds,  including  most  of  the  larger 
and  best-known,  the  green  is  mixed  up  with  red  or  brown  pig- 
ments which  aid  in  a  better  utilization  of  the  light  under  the  con- 
ditions prevailing  where  those  kinds  make  their  homes.  Their 
anatomical  structure  is  cellular,  as  in  land  plants,  but  much 
simpler,  with  far  less  division  of  labor  among  the  various  cells, 
and  only  unimportant  structural  differences  between  the  several 
tissues.  Their  reproduction  is  partly  by  fission,  but  chiefly  by 
spores,  which  are  simple  one-celled  bodies  various  in  aspect  and 
mode  of  formation, — some  of  them  actively  free-swimming,  and 
others  passively  floated  by  currents  of  water;  in  addition,  fertiliza- 
tion occurs,  in  all  grades  of  complexity  from  the  accidental  fusion 
of  two  precisely-similar  free-swimming  cells  up  to  the  union  of  a 
tiny,  free-swimming,  chemotropically-attracted,  male  cell  with  a 
sessile  food-filled  female  cell. 

The  best-known  kinds  of  the  Algae  are  these.  Among  the 
GREEN  (AND  BLUE-GREEN)  ALGAE  are  the  Diatoms, — found  in  all 
waters,  with  microscopical  flinty  shells  of  wonderful  beauty  and 
marvelous  variety;  the  Blue-green  kinds,  forming  unhealthy- 
looking  scums  of  that  color  in  unpleasant  damp  places;  Pleuro- 
coccus,  which  makes  up  the  familiar  green  coating  upon  the  shaded 
sides  of  standing  tree-trunks;  Vaucheria,  the  darker-green  coating 
on  damp  earth  in  warm  shaded  places;  Uroglcena,  hardly  visible  to 
sight,  which  gives  the  bad  odors  and  taste  to  the  water  of  reser- 
voirs, from  which,  fortunately,  it  can  be  driven  by  traces  of  com- 
pounds of  copper;  Spirogyra,  which  composes  the  very  bright  green 


Groups  into  Which  Plants  Naturally  Fall          447 

felted  mats,  buoyed  up  by  entangled  bubbles  of  gas  on  the  surface 
of  still  waters;  Cladophora,  and  its  relatives,  often  mistaken  for 
Mosses,  those  hair-like,  net-like,  brush-like,  fringe-like  forms 
which  sway  and  wave  from  their  moorings  on  stones  in  the  bot- 
toms of  slow-moving  brooks;  and  certain  of  the  simpler  kinds 
which  are  enslaved  in  the  meshes  of  some  Fungi  to  make  up  the 
remarkable  Lichens.  The  curious  Red  Snow,  reported  by  Arctic 
and  Alpine  expeditions,  and  the  redness  of  the  Red  Sea,  famed  in 
geography  and  biblical  history,  owe  their  characteristic  colors  to 
certain  red  stages  in  the  development  of  simple  Green,  or  Blue- 
green,  Algae. 

Of  the  BROWN  ALGAE  the  most  familiar  are  the  Rockweeds, 
whose  tough  branching  fronds  cover  rocks  of  the  beaches  where 
exposed  to  the  swing  of  the  tides;  the  great  leathery  Kelps, 
known  to  the  sailors  as  "Devil's  Aprons,"  abounding  in  the 
seas  of  the  north;  and  the  leafy-stemmed  kinds,  including  the 
Sargassum  which  gives  name  to  a  Sea,  more  plenty  towards  the 
south.  These  Brown  Algae  are  the  only  marine  kinds  which  are 
exposed  with  regularity  to  the  air,  either  between  tides  on  the 
beaches  or  during  notation  on  the  surface;  and  this  better  access 
to  gas-supply  helps  to  explain  their  larger  and  stouter  forms. 

Of  the  RED  ALGAE,  the  best  known  are  the  dark-red,  almost 
purple  Irish  Moss  and  Dulse,  familiar  to  all  persons  who  have  had 
the  good  fortune  to  grow  up  in  a  sea-port;  the  Corallines,  those 
reddish-chalky-warty  incrustations  upon  stones  near  low-tide 
mark,  often  mistaken  for  corals  which  they  aid  materially  in  the 
building  of  coral  reefs,  though  also  extending  far  north  of  the 
range  of  those  much  misunderstood  animals;  and  the  beautiful 
rose-red,  soft-foliaged  Sea-mosses,  most  plenty  towards  the  south, 
where  they  often  arouse  the  collecting  instinct  in  persons  who 
never  have  been  moved  to  collect  anything  else. 

Such  are  some  of  the  principal  ones  of  the  fourteen  thousand  or 
more  different  kinds  of  Algae  which  botanists  have  named  and 
described. 


448  The  Living  Plant 

As  one  might  expect,  there  are  plants  supposed  to  be  Algae 
which  really  are  not.  Thus  the  Eel-grasses,  the  Pond-weeds,  the 
Duck-weeds,  and  many  other  Water-weeds,  are  Flowering  Plants 
which  have  adopted  a  life  in  the  water,  and  therefore  an  Alga-like 
aspect.  They  can  be  told  by  their  flowers  which  they  bear  in 
their  season,  and  which  separate  them  sharply  from  the  spore- 
bearing  Algae. 

We  turn  now  to  consider  the  origin  and  evolution  of  these 
Algae,  together  with  their  classification.  If  evolution  is  a  fact, 
and  all  evidence  appears  to  agree  that  it  is,  then  classification 
must  be  an  expression  of  genealogical  descent,  and  expressible  in 
a  genealogical  tree,  comparable  with  the  kind  which  some  people 
are  fond  of  constructing  to  show  the  genealogical  ramifications  of 
human  families.  Such  a  tree,  for  the  great  primary  groups  and 
their  principal  subdivisions,  is  presented  in  our  accompanying 
diagram  (figure  177),  and  the  mode  of  its  construction  is  as  follows. 

First  as  to  its  most  ancient,  or  lowermost,  part.  We  have  good 
reason  for  believing,  as  the  chapter  on  Protoplasm  suggested,  that 
our  present  green  plants  were  preceded  in  time  by  a  colorless 
kind,  which,  though  without  chlorophyll  and  of  the  utmost 
simplicity,  could  yet  make  their  own  food  from  carbon  dioxide 
and  water  by  using  the  energy  of  chemical  oxidation  of  soil  min- 
erals in  place  of  that  of  the  sunlight.  We  have  precisely  such 
chemosynthetic  organisms,  a  kind  of  soil  Bacteria,  still  living  on 
the  earth  at  this  day;  and  they  are  doubtless  the  lineal  descend- 
ants of  the  ancient  forms,  which  probably  lived  in  the  mud  of 
shallow  seas  that  may  be  full  of  them  yet.  These  ancient  chem- 
osynthetic organisms  were  neither  animal  nor  plant  but  both  and 
between, — the  dawn  of  the  kind  of  plant-animal  forms  sometimes 
called  Protista;  and  therefore  I  suggest  that  we  call  them  Eo- 
protista.  These  Eoprotista,  therefore,  form  the  base  of  the  gen- 
ealogical tree.  Then,  like  all  later  groups,  they  must  have  ex- 
panded, developed,  varied,  evolved,  thus  originating  a  great 
many  branches,  of  which  the  greater  number  perished,  and  only 


Groups  into  Which  Plants  Naturally  Fall          449 

four  survived;  (a)  the  Chemosynthetic  Bacteria,  whose  persistence 
to  this  day  is  shown  by  the  continuous  line  sweeping  up  and  off  to 
the  outer  rim  of  the  tree  where  lies  the  vegetation  of  this,  our  own, 


Gamopetalous 
Dicotyledons 


Polypetalous 
Dicotyledons 

Apetalous  Trees 


Monocotyledons 


Basidia  Fungi 


Slime 
molds 


Eoprotista. 

FIG.  177. — A  genealogical  tree  of  the  principal  groups  of  plants.  The  axial  lines  show  the 
supposed  relations  of  the  groups  at  the  time  of  their  original  evolution  from  one  an- 
other, while  the  solid  trunks  show  their  present  numbers  and  connections. 

day;  (b)  the  Animals,  a  vast  group,  shown  on  the  left  by  an  un- 
finished stump  which  it  is  some  zoologist's  business  to  finish  if 
he  wants  it;  (c)  the  Slime-molds,  well  described  by  their  name,  a 
group  of  very  simple  organisms  which  creep  as  white  films  over 


450  The  Living  Plant 

damp  rotting  wood  in  dark  places,  in  a  way  so  like  to  some  animals 
that  zoologists  lay  even  stronger  claim  than  do  botanists  to  their 
possession,  and,  (d)  the  most  important  of  all,  the  Algae. 

So,  the  Algae  evolved  probably  from  Eoprotista,  and  by  a 
method  which  was  somewhat  like  this.  Among  the  variations  or 
mutations  (or  whatsoever  else  it  is  that  our  chapter  on  Evolution 
concluded  does  originate  innovations  in  living  Nature)  arising 
in  the  Eoprotista,  must  have  been  many  new  chemical  com- 
pounds, among  which,  in  time,  appeared  chlorophyll.  This  sub- 
stance happening  to  possess  such  properties  that  sunlight  falling 
upon  it  dissociates  carbon  dioxide,  enabled  its  possessors  to  make 
their  food  far  more  rapidly  and  easily  than  by  the  old  chemosyn- 
thetic  method;  and  therefore  those  plants  were  enabled  to  grow, 
increase,  develop,  and  expand  immensely  until  they  filled  the 
lighted  seas  of  the  world.  Thus  the  little  chlorophyll-bearing 
branch  of  our  tree,  the  one  that  happened  to  be  thus  fruitful 
among  so  many  that  were  barren,  expanded  so  greatly  that 
gradually  it  became  the  main  trunk  of  the  tree,  which  fact  we 
may  express  by  swinging  it  around  into  the  main  line  of  ascent  as 
has  been  done  in  our  diagram.  Thus  arose  the  Algae,  the  char- 
acteristic group  of  the  waters,  in  which  they  have  persisted  right 
down  to  the  present,  giving  origin  in  time  to  Red  and  Brown 
branches  as  the  tree  represents.  It  is  interesting  to  know  that 
our  living  Algae  have  an  ancestry  so  ancient,  so  ancient  indeed 
that  they  have  doubtless  had  time  to  evolve  everything  of  which 
they  are  capable,  and  have  consequently  reached  a  condition  of 
comparative  evolutionary  stability. 

The  Fungi. — These  are,  so  to  speak,  the  degraded  and  criminal 
classes  of  plants,  which  prey  upon  good  plant  society,  or  eke  out 
an  unenviable  existence  as  scavengers  of  its  offal.  Expressed 
more  precisely,  in  the  manner  of  science,  they  are  parasites  which 
take  all  their  food  ready-made  from  living  green  plants  or  from 
animals,  causing,  incidentally,  damage,  disease,  or  death;  or  else 
they  are  saprophytes  whch  consume  and  destroy  dead  animal  or 


Groups  into  Which  Plants  Naturally  Fall          451 

plant  remains,  thus  turning  them  back  into  the  general  circula- 
tion of  nature  and  rendering  a  service  to  the  remainder  of  living 
things.  This  dependence  upon  other  organisms  for  their  food, 
with  the  correlated  absence  of  chlorophyll,  is  then1  one  great  dis- 
tinctive feature. 

The  principal  kinds  of  Fungi  are  these: — Bacteria,  commonly 
called  "germs,"  or  "microbes,"  tiniest  of  living  things,  some  of 
them  harmless,  others  useful,  and  others  the  causes  of  deadly 
diseases;  Yeasts, — but  the  reader  knows  what  they  do;  Molds, 
which  spring  up  on  moist  bread,  preserved  fruits,  and  other  good 
materials,  spoiling  them  quickly  for  use;  Mycorhiza,  which  form 
caps  of  closely-felted  threads  over  the  ends  of  some  roots,  and  aid 
them  to  absorb  materials  from  the  soil;  Water-molds,  which  form 
the  white  haloes  round  dead  insects  or  small  fish  in  the  water; 
Blights  and  Mildews,  showing  as  powdery  or  woolly  white  fuzzes 
on  grape  leaves  and  others;  Rots,  which  soften,  discolor,  and  ruin 
potatoes  and  other  vegetables  and  fruits;  Spots,  which  darken 
round  areas  on  various  leaves;  Smuts,  which  convert  ears  of 
grain  to  an  unctuous  black  powder;  Rusts,  the  ragged  red  spots 
which  appear  on  the  leaves  of  the  wheat  in  over-wet  seasons,  and 
on  other  grains  also,  to  their  infinite  damage,  but  which  are  dear 
to  the  botanical  teacher  because  of  their  heterogeneously  poly- 
morphic ontogeny;  Mushrooms,  which  are  good  to  eat,  and  Toad- 
stools, which  are  not;  Puff-balls,  whose  names  sufficiently  describe 
them;  Black-knots,  which  form  swellings  on  branches  of  Cherries, 
with  many  destructive  diseases  of  Chestnuts  and  other  large 
trees;  Bracket-fungi,  which  appear  on  the  outside  of  tree-trunks 
as  a  kind  of  crude  hemispherical  bracket,  unfortunately,  however, 
with  the  flattened  side  down;  the  Lichens,  gray,  crisp,  brittle,  and 
crusted,  living  on  rocks,  fences  and  tree-trunks,  and  deriving  their 
food  from  certain  kinds  of  small  Algae  which  they  hold  enslaved 
in  their  meshes;  and  a  great  many  others  not  familiar  to  the 
public  but  well  known  to  botanical  students. 

In  shapes  the  Fungi  are  even  more  diversified  than  the  Algae, 


452  The  Living  Plant 

but  they  show,  for  the  most  part,  a  double  structure  imposed  by 
their  habit  of  life.  First,  they  possess  a  feeding  body,  called  a 
mycelium,  consisting  as  a  rule  of  innumerable  fine,  slender,  white 
threads  ramifying  and  radiating  everywhere  throughout  the 
accessible  tissues  of  the  living  plants,  or  amongst  the  decaying 
materials  upon  which  they  live;  and  second,  they  possess  a  com- 
pact spore-forming  body  which  comes  to  the  surface,  and  thus 
carries  the  spores  to  a  position  where  they  can  be  scattered  by  the 
wind.  Most  of  the  Fungi  familiar  to  us,  such  as  Rusts,  Bracket- 
fungi,  or  Mushrooms,  are  simply  the  spore-forming  bodies  of 
feeding  mycelia  which  branch  profusely,  though  invisibly,  through 
green  tissues,  tree-trunks,  or  earth.  And  it  is  an  interesting 
speculation,  by  the  way,  whether  kinds  like  the  Bacteria,  whose 
structure  and  habit  do  not  permit  them  to  bring  their  spores 
thus  to  the  surface  for  dissemination,  may  not  cause  the  death 
of  their  hosts  as  an  adaptive  measure  in  order  that  their  spores 
may  be  set  free  by  the  decomposition  of  their  victims.  The 
cellular  anatomy  of  the  Fungi  differs  in  a  curious  particular 
from  that  of  the  Algae  and  other  kinds  of  plants,  for  the  habit  of 
forming  the  thread-like  feeding  mycelium  persists  in  the  spore- 
forming  body,  which  is  simply  a  collection  of  compacted  and 
parallel  cellular  threads;  and  this  explains  why  it  is  that  Mush- 
rooms, for  instance,  break  apart  in  the  fibrous-grained  way  that 
they  do.  In  size  the  Fungi  are  all  rather  small,  ranging  from 
minute-microscopic  up  to  the  Toadstools  and  the  Bracket-fungi, 
which  never  exceed  some  two  feet  across,  though  to  the  size  of  the 
spore-forming  bodies  must  be  added  that  of  the  radiating  myce- 
lium, which  may  range,  albeit  tenuously,  over  a  diameter  of  several 
feet.  In  color  the  Fungi  are  not  green,  at  least  of  the  chlorophyll 
shade,  for  their  most  distinctive  feature  is  the  total  lack  of  that 
substance;  but  they  are  typically  white,  verging  to  gray  shades  or 
brown.  The  spore-forming  bodies,  however,  are  brilliantly 
colored  in  yellows,  purples,  and  reds  in  some  kinds,  notably  the 
Rusts  and  the  Smuts,  and  especially  some  of  the  poisonous  Toad- 


Groups  into  Which  Plants  Naturally  Fall          453 

stools,  though  it  is  not  yet  certainly  known  what  the  significance 
of  these  colors  may  be.  The  reproduction  of  the  Fungi  is  multi- 
farious, but  most  commonly  by  tiny  spores;  and  these  are  spread 
by  the  wind  with  the  dust,  of  which  they  make  up  no  inconsider- 
able part.  These  dust-like  spores  can  be  seen  en  masse  by  the 
reader  if  he  will  place  a  fresh  mushroom,  gills  down,  on  some  paper; 
for  after  a  few  hours  a  striking  spore-print  of  the  gills  will  appear. 
Spores,  furthermore,  are  often  of  a  thick-walled  "resting"  sort, 
which  can  endure  dryness,  heat,  light,  and  other  unfavorable 
conditions  for  months  or  even  years;  and  this  fact  helps  to  explain 
why  those  particular  plants  are  so  ubiquitous  and  irrepressible. 
But  they  also  reproduce  sexually,  at  least  some  of  them  do,  and 
usually  by  methods  so  closely  like  those  of  the  Algae  as  to  suggest 
a  relationship  between  these  two  groups.  This,  indeed,  is  a  con- 
clusion sustained  by  abundance  of  evidence;  and  it  all  goes  to 
show  that  the  Fungi  are  really  nothing  other  than  Algae  which 
have  taken  to  a  parasitic  habit  of  life. 

With  the  Fungi  are  commonly  reckoned  some  plants  which  are 
fungus-like,  but  not  Fungi.  Thus  the  Dodder,  and  the  Indian 
Pipe  are  Flowering  Plants,  though  they  have  no  chlorophyll  or 
leaves,  and  present  a  markedly  fungus-like  aspect  hi  correlation 
with  the  parasitic  or  saprophytic  habit  they  have  assumed.  And 
there  are  many  other  flowering  parasites  hi  all  degrees  of  develop- 
ment of  the  habit, — as  witness  the  Mistletoe,  which  is  only  a  part- 
parasite,  a  kind  of  a  natural  graft  which  takes  water  and  minerals 
from  its  host,  but  makes  its  own  food  by  means  of  its  own  chloro- 
phyll. Such  plants  can  always  be  told  by  then-  flowers,  which 
they  bear  at  some  time  in  their  lives,  and  which,  of  course,  are 
wholly  absent  from  the  Fungi. 

However  ignoble  the  habit  of  the  Fungi  may  appear  from  the 
view-point  of  green  plants  at  whose  expense  they  exist,  their 
manner  of  life  has  been  a  success;  for  it  has  enabled  them  to 
develop  no  less  than  some  sixty-six  thousand  different  kinds  al- 
ready known  and  described  by  Botanists  (between  four  and  five 


454  The  Living  Plant 

times  as  many  as  of  Algae),  while  there  doubtless  remain  a  great 
number  still  to  be  discovered. 

We  turn  now  to  consider  the  place  of  the  Fungi  in  our  tree  of 
descent  (figure  177).  It  seems  perfectly  clear  that  they  all  are 
derived,  either  immediately  or  remotely,  from  the  Algae.  We  can 
imagine  that  as  the  Algae  became  large  and  abundant,  some  kinds 
took  to  growing  upon  others,  at  first  merely  as  a  convenient  situa- 
tion, but  later  making  use  of  the  decaying  remains.  But  in 
nature,  as  in  human  affairs,  it  is  only  the  first  step  which  counts, 
and  the  transitions  from  a  dead  to  a  dying,  then  to  a  sickly,  and  fi- 
nally to  a  healthy  host  are  easy,  giving  origin  in  turn  to  an  epi- 
phytic, saprophytic  and,  finally,  parasitic  mode  of  life.  Then,  as 
the  Green  Algae  evolved  into  the  higher  and  air-living  forms  and 
came  out  to  live  on  the  land,  they  were  accompanied  by  these  par- 
asitic Algae,  which  gradually  became  more  and  more  altered  in 
adaptation  to  the  new  conditions  of  their  existence.  And  there 
you  have  the  Fungi,  which  are  nothing  but  parasitic  Algae,  al- 
though in  some  cases  with  an  ancestry  so  ancient  that  we  can 
hardly  trace  a  sign  of  their  primitive  origin.  The  various  principal 
sub-groups  of  the  Fungi, — the  Basidia  division  to  which  the  Mush- 
rooms belong,  most  ancient  and  specialized  of  them  all,  the  Sac 
Fungi,  which  include  the  Lichens,  the  Algoid  Fungi,  which  com- 
prise the  water  forms  and  others  that  are  most  like  the  Algae, — are 
shown  in  our  tree  in  conjunction  with  their  most  probable  an- 
cestral branches  of  the  Algae. 

The  Moss-plants,  or  Bryophytes. — These  are  typically  the  carpet 
plants  of  the  land,  especially  the  woods,  where  they  form  the 
fine  close  covering  over  ground,  boulders,  and  prostrate  tree- 
trunks;  but  they  also  extend  out  beyond  into  places  that  are  open, 
particularly  where  wet.  They  comprise  two  well-marked  divi- 
sions. First  are  the  Liverworts,  which  mostly  lie  flat  on  the 
ground  outspread  in  small  thin  fronds  suggestive  strongly  of  some 
kinds  of  Algae,  though  others  bear  delicate  leaves.  Second  are 
'the  true  Mosses,  much  more  familiar,  which  have  upright,  slender, 


Groups  into  Which  Plants  Naturally  Fall         455 

fine-leafy,  low  stems  growing  densely-compacted  together,  with 
slender-stalked  spore  cases  standing  out  from  then-  tops.  Most 
striking  of  them  all  are  the  Peat-mosses  (Sphagnum),  which  form 
in  wet  northern  climates  the  great  bogs  such  as,  doubtless, 
long  ago,  played  a  part  in  the  origination  of  the  coal  beds.  In 
size,  the  Moss-plants  are  all  low,  not  over  a  few  inches  in  height, 
and  they  have  no  parts  underground  excepting  some  water- 
absorbing  hairs, — which  fact  explains  why  all  Mosses  are  so 
easily  stripped  from  the  ground.  Their  cellular  anatomy  in  the 
best  developed  forms  includes  a  waterproof  epidermis  with 
stomata,  and  intercellular  spaces, — features  correlated  with  their 
air-living  habit ;  but  otherwise  the  tissues  are  little  more  special- 
ized than  in  Algae,  and  their  lack  of  a  particular  strengthening 
and  conducting  system  explains  why  they  never  can  rise  much 
above  the  ground.  In  color  they  are  typically  green,  often  intense 
in  its  shade,  from  the  presence  of  chlorophyll  with  which  they  all 
make  their  own  food,  though  the  greenness  is  often  obscured, 
especially  in  those  of  exposed  places,  by  screens  of  red  or  brown 
pigments  which  are  doubtless  a  protection  to  the  protoplasm 
against  the  injurious  action  of  untempered  light.  Their  reproduc- 
tion is  partly  by  dust-like  spores,  scattered  from  exposed  spore 
cases  by  the  wind,  and  partly  by  fertilization,  effected  by  the 
fusion  of  a  free-swimming  male  cell  with  a  well-enclosed  and  pro- 
tected egg-cell.  And  it  is  a  fact  of  great  interest  to  Botanists 
that  fertilization  and  the  production  of  spores  alternate  regularly 
with  one  another  in  two  separate  generations,  whereby  hangs  a 
remarkable  tale,  too  special  for  relation  in  this  place,  and  of 
which,  moreover,  the  exact  point  is  still  tantalizingly  elusive. 
As  to  the  numbers  of  the  Moss-plants,  some  seventeen  thousand 
kinds  have  been  described,  and  doubtless  a  good  many  are  still  to 
be  found. 

Of  course  there  are  plants  which  look  like  this  group,  but  are 
not.  Thus  there  is  a  " Liverwort"  which  is  a  Flowering  Plant 
(the  Hepatica),  while  the  "  Spanish  Moss,"  of  the  Oaks  in  the 


456  The  Living  Plant 

south,  is  also  a  Flowering  Plant  (belonging  to  the  Pineapple 
Family);  but  the  somewhat  similar  tree  moss,  or  "Long  Moss" 
of  the  northern  woods  ("The  murmuring  pines  and  the  hemlocks, 
bearded  with  moss  .  .  .  stand  like  Druids  of  eld")  is  a  Lichen, 
as  is  the  "Reindeer  Moss"  of  the  far  northern  plains.  The  "Sea- 
mosses"  are  Algae,  as  we  have  seen,  and  so  are  a  lot  of  the  moss- 
like  little  plants  of  fresh  waters.  Then  the  creeping  Ground  Pine 
of  our  woods,  known  even  botanically  as  a  "Club-moss,"  is  not 
a  Moss  at  all  but  a  Fern-plant,  of  the  group  next  to  be  studied. 
Furthermore,  even  Flowering  Plants,  especially  in  open  moun- 
tainous regions,  but  including  some  kinds  nearer  home,  like  the 
Pyxie,  assume  often  the  moss  habit,  and  therefore  the  moss  aspect, 
to  a  degree  often  completely  deceptive  were  it  not  for  their  tell- 
tale flowers  which  appear  at  some  season. 

We  turn  now  to  the  place  of  the  Moss-plants  in  our  tree  of 
descent  (figure  177).  There  is  no  question  as  to  their  origin  from 
the  Algae,  which,  among  a  great  number  of  branches,  must  have 
given  rise  to  one  with  a  structure  permitting  the  absorption  of 
gases  from  the  air  instead  of  the  water.  Thus  was  opened  up  to 
those  plants  an  immense  new  field  not  then  possessed  by  any 
other  plants  whatsoever, — all  the  surface  of  still  waters  and  the 
moister  parts  of  the  land, — which  latter  were  then,  it  is  likely,  far 
more  extensive  than  now.  Over  the  land,  accordingly,  these 
plants  proceeded  to  expand  as  a  dense  living  carpet,  then  the 
most  conspicuous  part  of  the  earth's  vegetation.  So,  our  diagram 
shows  their  particular  branch  swinging  into  the  main  trunk, 
thereby  displacing  the  Algae  to  a  lateral  limb ;  and  from  that  time 
to  the  present  these  Moss-plants  have  persisted  supreme  in  their 
own  situation,  giving  off,  however,  from  the  simpler  Liverworts 
the  more  complicated  Mosses. 

The  Fern-plants,  or  Pteridophytes. — These  are  typical  under- 
growth plants,  most  at  home  in  the  shade  of  the  woods,  where  they 
occupy  a  place  above  the  carpet  of  Moss-plants,  and  beneath  the 
canopy  of  the  forest,  though  like  all  other  groups  they  reach  far 


Groups  into  Which  Plants  Naturally  Fall          457 

out  beyond  their  own  particular  situation.  They  exhibit  three 
main  divisions.  First  are  the  true  Ferns,  whose  gracefully-cut 
fronds  and  general  habit  of  life  are  too  familiar  to  need  any  de- 
scription, though  the  reader  should  remember  that  in  the  tropics 
they  grow  into  trees,  among  the  most  beautiful,  though  not  the 
largest,  that  there  are.  Second  are  the  Horsetails,  which  are  stiff, 
green,  rush-like  plants,  with  terminal  spore-cones,  distinguished 
from  the  true  rushes  by  their  little  leaf  scales.  They  are  no  taller 
than  two  or  three  feet,  and  grow  mostly  in  shoal  water,  or  wet 
places,  but  sometimes  on  open  sandy  banks.  Third  are  the  Club- 
mosses,  creeping,  leafy,  and  not  unlike  their  namesakes,  the  true 
Mosses,  but  much  coarser,  as  the  common  Ground  Pine  well 
illustrates,  or  the  decorative  Selaginella  of  our  greenhouses;  while 
they  are  further  distinguished  by  their  little  terminal  cone-like 
masses  of  spore  cases.  In  size  all  three  divisions  of  the  Fern- 
plants  are  now  greatly  degenerate  from  a  former  high  estate,  for, 
along  with  others  now  extinct,  they  once  grew  into  the  trees  promi- 
nent in  the  earlier  geological  periods.  In  color  all  are  green  from 
the  chlorophyll  with  which  they  make  their  own  food,  and  no 
other  color  occurs,  save  an  occasional  red  blush  in  young  leaves, 
and  the  brown  of  their  spore-cases  or  stems.  Their  cellular  anat- 
omy is  well  differentiated  into  tissues  of  different  functions,  in- 
cluding a  highly-efficient  system  of  water-carrying  ducts  to- 
gether with  strengthening  fibers;  and  it  was  the  possession  of  this 
fibrovascular  system,  no  doubt,  which  permitted  these  plants  to 
carry  their  foliage  high  above  earth  upon  lofty  stems  from  deeply- 
anchored  roots,  thus  giving  the  world  its  first  forests.  Their 
reproduction  is  by  spores  spread  afar  by  the  wind  from  the  up- 
right plant,  and  this  spore-formation  alternates  with  fertilization 
which  occurs  in  a  way  and  a  place  not  suspected  by  most  persons. 
Thus  in  the  true  Ferns,  and  the  process  is  substantially  the  same 
in  principle  in  the  Horsetails  and  Club-mosses,  the  little  brown 
spores  from  the  under  sides  of  the  fronds  do  not  grow  into  plants 
like  those  which  produce  them,  but  into  small  (a  quarter-inch  in 


458  The  Living  Plant 

diameter)  thin,  filmy,  green,  prothallia,  lying  flat  on  the  ground 
in  wet  places  and  strongly  suggesting  either  Liverworts  or  Algae. 
On  their  under  sides  are  well-protected  egg-cells  fertilized  by  male 
cells  which  swim  freely  through  water  caught  under  the  prothal- 
lium.  Then  from  this  fertilized  egg-cell  arises  the  familiar  Fern- 
plant  ;  and  we  have  here  a  very  perfect  example  of  that  alternation 
of  generations  which  is  of  such  great  botanical  interest.  But  it  is 
evident  that  the  Fern-plants  are  dependent  for  their  fertilization 
upon  the  presence  of  standing  water,  though  this  can  be  supplied 
by  a  flooding  during  rain-storms;  and  this  is  the  reason  why  those 
plants  are  confined  for  the  most  part  to  shaded  or  moist  places. 
As  to  their  numbers,  some  three  thousand  five  hundred  different 
kinds  are  known,  with  doubtless  not  a  great  many  more  to  be 
found. 

With  the  Fern-plants  are  commonly  reckoned  a  good  many 
others  which  do  not  belong  there.  Indeed,  to  most  people,  any 
plant  with  finely-cut  foliage  is  thereby  made  a  Fern,  though  many 
such  plants  will  be  found  to  flower  at  intervals.  The  little 
Japanese  "Air-plant,"  graceful,  feathery  and  deceptively  Fern- 
like,  is  in  fact  an  animal  production,  the  tough  horny  skeleton  of 
a  little  marine  Hydroid,  so  naturally  stained  and  arranged  that 
not  a  few  people  declare  they  have  witnessed  it  grow! 

We  turn  now  to  the  place  of  the  Fern-plants  in  our  tree  of 
descent  (figure  177).  All  evolutionary  analogy  would  show  that 
the  Moss-plants  like  all  other  groups,  gave  off  many  branches,  of 
which  one  in  particular  was  a  brilliant  success.  It  was  the  branch 
which  developed  a  vascular  system  permitting  the  ready  conduc- 
tion of  water;  and  this  freed  those  plants  from  their  old  ground- 
clinging  habit  and  opened  to  them  the  upper  air  for  the  spread 
of  great  masses  of  foliage  to  the  sun.  Thus  arose  the  Fern- 
plants,  the  earliest  trees,  which  spread  over  the  moister  earth  as 
its  dominant  vegetation,  a  fact  which  our  tree  represents  by  the 
swinging  of  this  branch  into  the  main  trunk,  displacing  the  Moss- 
plants.  And  they  have  persisted  to  the  present  in  their  own 


Groups  into  Which  Plants  Naturally  Fall          459 

situations,  though  sadly  diminished  in  number  and  size,  and  re- 
duced to  the  position  of  undergrowth,  by  the  insistent  and  success- 
ful competition  of  a  still  higher  group,  the  Flowering  Plants. 
The  three  divisions  they  have  developed  are  also  shown  by  the  tree. 

The  Flowering  Plants,  called  also  Seed-plants,  or  Spermato- 
phytes. — These  are  all  the  rest  of  the  plants  of  the  earth,  com- 
prising all  of  the  loftiest  trees,  practically  all  of  the  shrubs,  and 
the  innumerable  flower-bearing  herbs  no  matter  where  found, 
whether  in  woods,  fields,  waters,  plains,  mountains,  deserts,  or 
sea-shores.  In  shapes  they  are  manifold,  though  usually  dis- 
playing the  characteristic  differentiation  into  root,  stem,  leaf, 
flower,  and  fruit,  the  functions  of  which  are  now  well  known  to 
the  reader;  but  these  parts  may  be  modified  multifariously  in 
form,  size,  and  combinations  in  adaptation  to  particular  condi- 
tions of  life.  In  size  they  range  from  the  Redwoods,  over  three 
hundred  feet  high  and  thirty  feet  through,  down  to  some  Duck- 
weeds, hardly  larger  than  the  head  of  a  pin.  In  color,  since  they 
make  their  own  food,  they  are  typically  green  from  the  presence  of 
chlorophyll,  though  some  have  become  parasites  and  lost  it;  but 
in  some  special  parts,  notably  flowers  and  fruits,  they  have  de- 
veloped well-nigh  all  the  shades  of  the  rainbow  in  adaptation  to 
the  accomplishment  of  particular  functions.  In  their  cellular 
structure  they  are  developed  beyond  all  other  groups  in  special- 
ization and  division  of  labor,  which  is  a  reason  for  their  obvious 
and  growing  dominance  in  all  situations.  Then*  reproduction  is 
chiefly  through  seed-formation  (whence  the  name  of  the  group), 
following  upon  the  fertilization  of  an  egg-cell  in  the  ovule  by  a 
male  cell  brought  by  a  pollen-tube,  as  already  very  fully  described 
in  our  chapter  on  Reproduction. 

This  fertilization  arrangement,  whereby  a  male  cell  is  carried 
by  a  tube  from  a  pollen  grain  to  an  egg-cell  borne  high  on  a  plant, 
seems  at  first  sight  to  possess  nothing  in  common  with  that  in  the 
Fern-plants,  where  the  male  cell  swims  freely  to  the  egg-cell 
through  water  caught  under  a  prothallium  pressed  close  to  the 


460  The  Living  Plant 

ground.  But  in  fact  there  is  every  gradation  between  them,  and 
one  answers  morphologically  to  the  other  in  a  manner  most 
striking  and  satisfactory,  though  it  is  not  any  part  of  my  business 
at  present  to  explain  the  matter  more  fully.  But  there  is  one 
thing  about  this  pollen-tube  arrangement  that  is  of  greatest 
evolutionary  importance, — viz.,  it  has  rendered  these  plants  in- 
dependent of  standing  water  and  a  prothallium  on  the  ground  for 
their  fertilization,  and  has  thus  freed  them  from  the  restriction 
which  limits  the  range  of  the  Fern-plants.  Hence  the  Flowering 
Plants  are  able  to  extend  over  places  too  dry  for  the  Fern-plants, 
and  indeed  over  all  parts  of  the  earth  where  plant-life  is  a  possi- 
bility at  all;  and  not  only  that,  but  through  virtue  of  their  higher 
organization  in  other  respects  they  are  able  to  compete  with  the 
lower  groups, — the  Undergrowth  plants,  the  Carpet  plants,  the 
Parasitic  plants,  and  the  Water  plants, — in  their  own  peculiar 
situations,  of  which  they  are  slowly  but  surely  taking  possession 
in  the  course  of  their  evolution.  And  the  best  evidence  of  their 
success  is  found  in  their  numbers,  for  they  have  been  able  to 
develop  no  less  than  some  one  hundred  and  thirty-three  thousand 
distinct  species  already  known  and  named,— many  more,  it  will 
be  noted,  than  of  all  the  other  groups  put  together. 

The  Flowering  Plants  include  two  very  distinct  groups.  First 
are  the  Gymnosperms, — Pines,  Spruces,  Firs,  and  that  sort, — 
which  are  trees  and  tall  shrubs  without  any  flowers,  and  bearing 
their  seeds  naked  on  the  branches,  or  partly  covered  by  cone- 
scales;  and  they  are  almost  wholly  wind-disseminated  and  wind- 
pollinated.  Second  are  the  Angiosperms,  with  their  seeds  en- 
closed always  in  an  ovary  which  is  part  of  a  flower.  Some  of 
them, — the  Oaks,  Chestnuts,  Beeches,  Elms,  Birches,  Alders,  and 
such  kinds, — are  trees  or  tall  shrubs,  wind-pollinated  (and  there- 
fore without  showiness  in  the  flowers)  and  wind-disseminated. 
The  remainder  fall  into  two  sub-groups,  Dicotyledonous  or  Ex- 
ogenous Plants,  which  appear  to  occupy  the  main  line  of  advance, 
and  Monocotyledonous  or  Endogenous  Plants,  which  seem  to 


Groups  into  Which  Plants  Naturally  Fall          461 

have  been  separated  from  the  Dicotyledons  through  an  early 
partial  return  to  a  water  habit.  Both  of  these  sub-groups  are 
distinguished  for  the  most  part  by  insect-pollination,  with  its 
correlated  floral  showiness;  and  so  much  more  effective  and 
economical  is  this  insect-pollination  than  wind-pollination  that 
the  Flowering  trees, — Locusts,  Magnolias,  and  most  Fruit  trees, — 
are  slowly  driving  the  wind-pollinated  kinds  from  the  earth. 
This  insect-pollination,  moreover,  with  which  naturally  goes 
animal-dissemination,  renders  the  plants  independent  of  exposure 
to  winds  for  both  pollination  and  dissemination,  and  hence  capable 
of  growing  in  all  kinds  of  retired  and  lowly  situations.  Therefore 
there  exist  not  only  Flowering  shrubs,  which  can  grow  as  under- 
growth in  successful  competition  with  the  Fern-plants,  but  also 
Flowering  herbs,  which  can  grow  in  all  sorts  of  places,  even  in 
competition  with  the  carpeting  Moss-plants,  with  the  Water- 
plants,  and  with  the  Parasites. 

We  turn  now  to  the  place  of  the  Seed-plants  in  our  tree  of 
descent  (figure  177).  Among  the  branches  produced  by  the 
Fern-plants  must  have  been  one  with  a  wind-carried,  tube- 
producing,  pollen-grain, — a  discovery,  or  invention,  which  ren- 
dered its  possessors  independent  of  the  standing  water  needed  by 
the  Fern-plants  for  fertilization,  thus  enabling  them  to  range  far 
more  freely  over  the  earth.  Such  was  the  origin  of  the  Seed- 
plants,  which  swung  into  the  main  line  of  dominance,  where  they 
persist  to  this  day.  The  first  kinds  were  undoubtedly  trees,— 
Gymnosperms  and  wind-pollinated  Angiosperms, — whose  exact 
relations  to  one  another  are  still  very  uncertain;  but  from  the 
latter  originated  the  insect-pollinated  kinds,  first  trees,  then 
shrubs,  then  herbs.  These  latter  possess  all  of  the  advantages  of 
the  lower  groups  in  addition  to  their  own, — are  the  heirs  of  all 
the  ages  in  fact ;  and  their  higher  organization  is  permitting  them 
to  do  precisely  the  same  thing  that  the  higher  races  of  men  are, — 
to  take  possession  of  the  earth  to  the  suppression  and  extinction 
of  the  lower  races. 


462  The  Living  Plant 

Such  are  the  five  primary  groups,  sometimes  called  Classes,  of 
Plants.  Each  is  divided  into  sub-groups  called  Orders,  and  those 
again  into  others  called  Families,  and  those  again  into  others 
called  Genera,  and  those  into  Species.  It  is  theoretically  possible 
to  follow  out  the  branches  of  our  genealogical  tree  through  smaller 
and  smaller  ramifications  to  the  ultimate  tips,  representing  the 
species,  of  which  there  would  be  some  two  hundred  and  fifty 
thousand ;  and  the  construction  of  such  a  tree  is  the  aim  of  every 
student  of  classification.  It  is,  however,  no  part  of  our  present 
business  to  follow  this  matter  any  farther,  for,  while  the  primary 
groups  are  distinguished  very  largely  by  differences  of  habit,  this 
becomes  less  and  less  true  with  the  groups  that  are  smaller,  and 
hardly  at  all  with  the  species,  which  are  mostly  marked  off  from 
one  another  by  characters  having  little  connection  with  adapta- 
tion. 

The  Flowering  Plants  are  the  highest  yet  developed  within  the 
Plant  Kingdom.  Are  there  then  no  higher  possibilities  in  plant 
evolution?  So  far  as  concerns  any  new  field  for  them  to  expand 
in,  there  seems  to  be  none,  unless  they  follow  the  example  of  man, 
and  take  to  free  flight  in  the  air.  But  the  world  is  not  yet  finished, 
nor  are  all  the  possibilities  of  variational  experimentation  ex- 
hausted; and  until  such  times  come,  evolution  is  not  likely  to 
cease. 

There  remains  one  other  aspect  of  classification  to  be  men- 
tioned before  this  chapter  can  be  finished.  Although  the  large 
genealogical  groups  we  have  been  considering  happen  to  be  dis- 
tinguished pretty  well  from  one  another  in  habit,  and  thus  con- 
stitute also  ecological  groups,  the  correspondence  between  gen- 
ealogy and  ecology  is  by  no  means  exact.  Examples,  indeed,  of 
the  ecological  intrusion  of  the  genealogical  groups  into  one  an- 
other have  been  given  in  the  preceding  pages;  and  further  study 
only  serves  to  increase  the  number  of  such  cases.  Every  group 
is  striving  to  expand  to  its  utmost,  and  whenever  it  can  find  an 
unoccupied  crevice  in  the  territory  of  another,  it  is  not  deterred 


Groups  into  Which  Plants  Naturally  Fall          463 


Strand-plants 
(Balophytw) 


FIG.  178.— A  diagram  showing  the  mutual  interrelations  of  the  genealogical  and  ecological 
groups.  The  widths  of  the  connecting  bars  show  the  approximate  number  of  species 
involved. 


464  The  Living  Plant 

by  any  genealogical  courtesies  from  expanding  to  fill  it.  The  re- 
sult is  this,  that  kinds  of  plants  genealogically  related  have  come 
to  acquire  very  different  habits,  and  hence  to  belong  to  very 
different  ecological  groups,  while  the  different  ecological  groups 
include  many  kinds  having  the  most  different  genealogical  rela- 
tionships, a  matter  which  is  brought  out  diagrammatically  in  the 
accompanying  figure  (figure  178).  It  is  with  plants  as  with  men, 
who  may  be  grouped  by  their  blood  relationships  into  families  or 
clans  on  the  one  hand,  or  according  to  their  occupations  into 
trades,  businesses,  or  professions  on  the  other.  Sometimes  the 
two  arrangements  overlap,  especially  among  primitive  peoples, 
but  often  they  do  not,  particularly  in  the  higher  civilizations. 
These  ecological  groups  of  plants  have  been  characterized  more  or 
less  fully  in  the  preceding  pages,  and  need  only  be  summarized 
very  briefly  at  this  place. 

A  SYNOPSIS  OF  THE  ECOLOGICAL  GROUPS  OF  PLANTS 

I.  INDEPENDENT  PLANTS,  or  AUTOPHYTES,  the  highest  and  most 
distinctive  plants,  making  their  own  food  by  aid  of  chlorophyll,  and  in- 
cluding: 

1.  NORMAL  PLANTS,  or  MESOPHYTES,  living  rooted  in  aerated  soil  sup- 
plying enough  moisture  to  permit  a  wide  spread  of  leaves  and  stems; 
mainly  Flowering  plants,   but  with  many  Fern-plants  and  Moss- 
plants,  commonly  massed  together  into  forests  which  exhibit  a  canopy 
of  trees,  an  undergrowth  of  shrubs,  and  a  carpet  of  herbs. 

Furthermore,  some  kinds  of  Mesophytes  are  so  strongly  adapted 
to  some  particular  condition  of  life  as  to  rank  as  separate  groups, — viz., 
Air  plants,  or  EPIPHYTES,  including  members  of  all  the  genealogical 
groups,  growing  supported  upon  other  plants,  and  highly  adapted 
to  that  peculiar  habit:  CLIMBERS,  mostly  Flowering  plants,  whose  very 
slender  stems,  lean,  cling  or  twine  by  aid  of  others  up  to  the  light: 
TRAILERS,  of  all  groups,  which  keep  flat  on  the  ground  as  a  part  of  the 
carpet:  INSECTIVOROUS  Plants,  wholly  Flowering  plants,  which  sup- 
plement the  scantness  of  soil  nitrogen  in  the  places  where  they  live  by 
capturing  and  digesting  insects  through  aid  of  remarkable  adaptations: 
MYRMECOPHILOUS  Plants,  Flowering  plants  of  the  Tropics,  supposed 
to  attract  ants  for  protection  against  other  insect  enemies,  but  of  doubt- 
ful ecological  status  at  present. 

2.  WATER  PLANTS,  or  HYDROPHYTES,  living  largely  immersed  in  water 
from  which  they  take  their  minerals  and  gases,  and  therefore  mostly 
soft-bodied  and  finely  divided;  mainly  Algae,  but  including  some  Moss- 
plants,  Fern-plants,  and  Flowering  plants. 


Groups  into  Which  Plants  Naturally  Fall          465 

3.  STRAND  PLANTS,  or  HALOPHYTES,   living  along  the  margin  of  salt 
water,  and  therefore  condensed  and  otherwise  adapted  to  the  difficult 
absorption  thereof;  a  few  Flowering  plants  only. 

4.  DESERT  PLANTS,  or  XEROPHYTES,   living  hi  places  excessively  dry, 
and  therefore  condensed  and  protected  for  water  conservation;  mainly 
Flowering  plants,  with  a  few  Lichens. 

II.  DEPENDENT  PLANTS,  sometimes  called  HYSTEROPHYTES,  in- 
cluding PARASITES  and  SAPROPHYTES,  which  take  their  food  from 
other  organisms,  either  living  or  dead,  and  lack  chlorophyll  and  leaves; 
mainly  Fungi,  but  including  some  Flowering  plants. 

Finally,  there  is  one  more  way  in  which  plants  are  classified 
ecologically.  When  considered  en  masse,  plants  constitute  vege- 
tation, and  vegetation  can  be  classified.  A  mass  of  vegetation 
which  gives  a  distinctive  aspect  to  a  country,  such  as  a  Pine 
forest,  or  a  natural  meadow,  is  called  a  Formation;  any  group  of 
plants  commonly  occurring  together  therein  is  called  an  Associa- 
tion; while  the  word  Society  is  somewhat  loosely  used  for  any  kind 
of  vegetation  group.  This  subject  is  one  very  much  studied  at 
present,  and  will  ultimately  give  us  a  vivid  method  of  describing 
causally  the  vegetation  of  any  country. 


INDEX 


Figures  in  heavy-faced  type  indicate  illustrations 


Abnormal  growths,  357 

Absorption,  water,  165;  system,  168,  268; 

machinery,  169;  by  roots,  172;  cortex 

to   ducts,    174;   of  minerals,    189;   of 

gases,  190;  of  organic  substances,  193; 

of  salt  water,  268 
Acacias,  70 
Acanthus,  385 
Accident  in  growth,  369 
Acclimatization,  254 
Accumulation  of  characters,  409 
Acids,  117;  chemistry,  118 
Adaptation,  nature,  vi,  vii,  11,  12,  47,  60, 

424 

Adjustive  vs.  adaptive  structures,  254 
Adjustment  of  plants  to  light,  224;  to 

moisture,  239;  to  chemical  substances, 

241;  to  touch,  242;  to  gravitation,  245; 

to  minor  influences,  251;  in  growth, 

369 
Aeration;  system,   190,    191,   192,   206, 

271;  in  water-plants,  194;  of  soils,  85 
Aerial  roots,  68,  382 
Aerotropism,  241,  242 
Aging  of  plants,  163 
Agriculture,  3 
Albumins,  127 
Alcohol,  formation,  97,  98 
Algae,  of  hot  springs,  265;  fertilization, 

304;  described,  445 
Algal  paper,  377 
Alkaloids,  103,  106,  125,  274 
Alpine  plants,  321 
Alterability  of  individuals,  413 
Alternation  of  generations,  455 
Amides,  106,  120,  125 
Amoeba,  380 
Amygdalin,  119,  120 
Anatomy,  defined,  2 


Anchor  roots,  196,  358 

Angiosperms,  460,  461 

Animals,  protection  against,  274;  pol- 
lination by,  309;  dissemination  by, 
394,  461 

Annuals,  366 

Annual  rings,  365 

Anther,  281 

Anthocyan,  39,  119 

Antibodies,  254 

Ants,  and  flowers,  325;  and  dissemina- 
tion, 398 

Aphorisms,  of  Bacon,  9 

Applications  of  science,  4,  5 

Aristolochia,  fertilization,  311 

Ascent  of  sap,  213 

Asparagin,  120 

Association,  465 

Atmospheric  pressure,  215 

Autophytes,  464 

Autumn  coloration,  described  and  ex- 
plained, 40,  Plates  II,  III;  significance, 
43;  causes  influencing,  45,  148 

Autumn,  vegetation  in;  368 

Auxograph,  329,  330 

Bacteria,  4,  102,  122,  262,  273,  281,  391, 
451;  chemosynthetic,  448;  dissemina- 
tion, 452 

Bacteriology,  3 

Barometric  pressure  and  growth,  336 

Basal  food,  106 

Bayberry  wax,  118 

Bezoars,  377 

Biennials,  explained,  366 

Biology,  defined,  3 

Biotypes,  420,  423 

Birds,  in  cross  pollination,  323;  in  dis- 
semination, 395,  397 


467 


468 


Index 


Birdseye  Maple,  372,  373 

Birth-rate  in  man,  302 

Black-knots,  451 

Bladders,  on  fruits,  388 

Blades,  of  leaves,  70 

Blights,  273,  451 

Blue-green  Algse,  380,  448 

Body  plasm,  415 

Boston  Ivy,  13,  232 

Botanical  study,  1 ;  education,  3 

Botany,  defined,  2 

Bracts,  70 

Bracket-fungi,  451,  452 

Branching,  explained,  49 

Bread  raising,  98 

Breathing,  85,  191 

Brown  Algae,  447 

Brown  in  leaves,  44 

Browning  of  evergreens,  202 

Bryophytes,  454 

Buds;  originate  stems,  61;  coverings,  67; 

protection  of,  277;  development,  359, 

360;  position,  361 
Bud  sports,  437 
Bulblets,  279 
Burbank,  443 
Burdock,  395 

Bursting  of  pavements,  79 
Butcher's  Broom,  71 

Cactus,  68,  70,  212,  264,  269,  277, 
396 

Caffeine,  125 

Cambium,  155,  221,  362 

Camphor,  117 

Cane  sugar,  chemistry,  27,  108 

Capillarity,  169;  phenomena,  179;  ex- 
plained, 179;  diagram,  179,  180;  in 
plant  life,  181,  215 

Carbohydrates,  106,  107;  derivatives, 
106,  116 

Carbon  dioxide,  in  atmosphere,  29;  in 
photosynthesis,  30,  31,  35,  48;  in 
respiration,  81 

Carbonization,  113 

Cardinal  points,  333 

Carpet  plants,  454,  460 

Caruncle,  398 


Cell  division,  284,  287,  341,  355 

Cells,  described,  20,  160;  sugar  holding, 
107;  conventionalized,  161;  shapes, 
153,  164;  discovery,  158;  named,  158; 
why  exist,  161;  sizes,  161;  enlarge- 
ment, 342,  343 

Cellulose,  described,  111;  chemistry,  113; 
modifications  of,  113;  alteration  to 
coal,  113,  157 

Cell  wall,  150,  151;  shapes,  153;  thick- 
ness, 166;  composition,  157 

Chemosynthetic  Bacteria,  448,  449 

Chemotropism,  241 

Chimseras,  351 

Chlorophyll;  distribution,  17,  18;  flu- 
orescence, 19;  properties,  19;  insta- 
bility, 19;  grains,  21,  22,  110,  Plate  I; 
and  light,  32,  34;  in  photosynthesis,  35; 
why  green,  37;  composition,  117;  origin 
in  time,  450 

Chloroplastids,  160 

Chondriosomes,  159 

Chromatin,  284 

Chromosomes,  in  division,  77,  284,  287; 
composition,  129,  160,  162 

Chrysanthemum,  improvement  of,  426, 
427 

Cion,  349 

Circulation  of  substances  in  Nature,  133 

Circumnutation,  77,  346,  348 

Cladophora,  447 

Classes  of  plants,  462 

Classification,  2,  404,  445 

Cleistogamous  flowers,  316,  318 

Clematis  seed,  389 

Clerodendron  flowers,  316 

Climbers,  242,  464 

Climbing  organs,  72 

Clinostat,  238,  239 

Close  pollination,  438 

Clover,  fruits,  387 

Club-moss,  456,  457 

Clusters  of  flowers,  explained,  321 

Coal,  formation,  114 

Cobcea  scandens,  326 

Cocaine,  125 

Cockscomb,  373 

Cocoanut,  dissemination,  394 


Index 


469 


Color  17,  37;  in  seaweeds,  38,  446;  in 
foliage  plants,  38;  in  spring  vegeta- 
tion, 39;  screens,  39;  in  autumn  leaves, 
40;  Plates  II,  III;  in  cross  pollination, 
308,  310,  319,  320;  contrasts,  318; 
variegation,  321;  changes,  321;  of 
fruits,  398;  in  Fungi,  452 

Compass  Plant,  232,  264 

Correlation  in  growth,  372 

Combustion,  89,  114 

Composite  conceptions,  7,  9 

Compounding  of  leaves,  59 

Compromises  in  structure,  223 

Conduction,  145;  system,  168 

Cone  shapes  of  trees,  56,  58,  260 

Consciousness,  255 

Continuity  of  protoplasm,  157,  158 

Conventional  constants,  explained,  9; 
26,  28,  31 

Copper  beeches,  38,  436 

Corallines,  447 

Cork,  155 

Corn,  silk,  307 

Cortex,  222 

Cotton  seed,  389 

Cotyledons,  354 

Cross  fertilization,  294;  advantage  of, 
295;  in  water  plants,  303 

Crossing,  437 

Cross  pollination,  303;  vs.  cross  fertili- 
zation, 303;  by  water,  304;  by  wind, 
305;  by  insects,  309;  books  on,  314; 
arrangements  to  ensure,  315;  by  birds, 
323;  by  snails,  324,  438 

Crystals,  134,  274 

Cultivation,  429,  442 

Cup  leaves,  375 

Curbstones  lifted,  78 

Cutin,  113,  157 

Cut-leaf  forms,  436 

Cycles,  in  life,  163;  in  growth,  352 

Cytase,  128 

Cytology,  denned,  2 

Cytoplasm,  150,  159 

Dandelion  seed,  389 

Darkness,  and  plants,  91,  334 

Darwin;  letter,  6;  11,  12,  234,  266,  294, 


314,  346,  347,  349,  406,  409;  biography, 
411,  417,  419,  423 

Date  seed,  112,  156 

Death;  144,  163;  cause  in  trees,  367 

Decay,  described,  102 

Deciduous  trees,  shape,  51,  261,  262 

Denatured  alcohol,  99 

Desert  plants,  characters,  203,  264,  465 

Development,  327,  340,  353 

De  Vries,  418,  420,  423 

Dextrose,  27;  chemistry,  107 

Diagrams,  nature,  9 

Diastase,  110,  128 

Diatoms,  380,  420,  446 

Dichogamy,  315 

Dicotyledons,  460 

Diffusion,  nature  of,  175;  diagram  of, 
176;  of  gases,  193,  206,  218 

Digestion,  110,  218 

Dimorphism,  316,  317 

Diseases,  102 

Dissemination,  116,  118,  279,  378;  rea- 
sons for,  378;  methods,  379;  locomo- 
tion, 379;  vs.  cross  pollination,  379; 
by  growth,  381;  by  shortening  roots, 
382;  by  projection,  383;  by  hygro- 
scopic movements,  385;  by  gravita- 
tion, 387;  by  winds,  387;  by  reduction 
in  size,  390;  by  water,  393;  by  animals, 
394;  by  man,  399;  minor  adaptations, 
401;  books  on,  401 

Distillation,  99 

Diversity  of  plants,  16 

Division,  145,  280 

Dixon  and  Joly,  216 

Dodder,  453 

Dominance,  296 

Double  fertilization,  300,  356 

Double  flowers,  375,  436 

Double  fruits,  273 

Drainage,  87,  201 

Drains,  filled  by  roots,  241 

"Drawing"  of  plants,  334,  431 

Dreams,  357 

Drowning  of  roots,  87 

Dry-blasting  of  plants,  202 

Dryness,  protective,  266;  protection 
against,  267 


470 


Index 


Dry  weights,  344 
Duckweeds,  393,  448,  459 
Ducts,  155,  213,  221 
Dulse,  447 
Dust,  and  spores,  391 

Ecology,  denned,  3;  vs.  morphology,  73 

Ecological  groups,  463,  464 

Economic  Botany,  3 

Edible  fruits,  397 

Eel  grasses,  195,  306,  448 

Egg  cell,  281 

Electricity  and  growth,  336 

Electrolysis,  35 

Electrotropism,  251 

Elementary  species,  420 

Emaciation,  91 

Embryo,  354 

Embryology,  denned,  2,  405 

Embryo  sac,  281 

Endogenous,  vs.  exogenous,  364;  461 

Endosperm,  300,   354 

Energy  in  plants,  79;  source,  80,  po- 
tential vs.  kinetic,  93 

Enzymes,  106,  126;  principal  kinds, 
128;  catalyzers,  129;  in  digestion, 
217 

Eoprotista,  448 

Epidermis,  155,  222;  of  desert  plants, 
269,  270 

Epiphytes,  464 

Erythrophyll,  described,  39;  uses,  39; 
in  autumn  leaves,  42;  119 

Essential  oils,  117 

Ether  and  growth,  336 

Ethereal  oils,  117 

Eucalyptus,  201 

Euphorbia  splendens,  68 

Evergreens,  shape,  68,  259 

Evolution,  403;  vs.  special  creation,  403; 
evidence,  404;  explanations,  404;  a 
scientific  question,  405;  by  natural 
selection,  406;  diagram,  410;  and 
Darwin,  411;  and  Lamarck,  412;  by 
transmission  acquired  characters,  413; 
and  Weismann,  415;  epochs,  417;  and 
de  Vries,  418;  by  mutation,  418;  and 
Mendel,  422;  new  indications,  423; 


under  experiment,  425;  vs.  improve- 
ment, 429 

Excretion,  of  gases,  211;  of  minerals, 
212;  of  root-poisons,  212;  of  nectar, 
212;  of  nitrogen,  125 

Exogenous  vs.  endogenous,  364;  460 

Experiment,  use,  22,  31 

Experiment  greenhouse,  8 

Explosion,  of  fruits,  384;  of  stamens,  323 

Experimental  evolution,  418,  425 

Extra-floral  nectar,  212 

Fairy  Rings,  381,  382 

Fasciations,  373,  374 

Female,  meaning  of,  291 

Fermentation,  described,  97;  economics, 
98;  chemistry,  99;  object,  100;  relation 
to  respiration,  100,  101;  by-products, 
101 

Ferments,  128 

Ferns,  457;  fertilization  of,  241,  283,  289, 
290;  304 

Fern-plants,  described,  456 

Fertilization,  described,  281,  283,  285, 
286,  437,  459 

Fibres,  221, 222 

Fibrovascular  bundles,  221;  363,  364, 
365;  tissues,  457,  458 

Fission,  280 

Flesh-formers,  106,  126 

Flotage  of  seeds,  393 

Flowering  Plants,  described,  459 

Flowers,  morphology,  69;  turn  to  light, 
235;  236;  protection  of,  276;  geotro- 
pism,  249;  described,  281,  282;  pecul- 
iarities explained,  310:  defined,  310; 
correlations  with  insects,  317;  essential 
parts,  362;  green,  375 

Flowers  of  tan,  141 

Fluctuating  variations,  423 

Foliage  plants,  38,  428 

Food,  function,  94;  a  storage  battery,  96; 
reservoirs,  68;  106, 107 

Forestry,  3 

Fortuity  of  variation,  433 

Fossils,  404 

Freaks,  373,  436 

Frost,  and  autumn  colors,  45 


Index 


47* 


Frost  weeds,  211 

Fructose,  27;  108 

Fruit  Jellies,   115 

Fruits,  turn  to  light,  235;  from  light, 
228,  235;  protection  of,  276;  378;  func- 
tion, 391;  vs.  seed,  391 

Fruit  sugar,  chemistry,  27,  107 

Fungi,  described,  450 

Galls,  370 

Gases,  absorption,  190 

Gelatine,  115 

Genealogical,  trees,  448,  449 ;  groups,  463 

Generalizations,  7 

Generalized  drawings,  nature,  9 

Generalized  knowledge,  9 

Genotypes,  298,  423 

Geographical  distribution,  404 

Geotropism,  245;  of  stems,  246;  of  roots, 

228,  245;  of  leaves,  248;  of  flowers,  248; 

of  fruits,  250;   transverse,    246,  250; 

lateral,  250;  correlation  of,  373 
Germination,  358 
Germ  cells,  415,  421 
Germ  plasm,  415 
Gibson,  315 
Globulins,  127 
Glucosides,  119,  274 
Glutelins,  127 
Gradations,  404 
Grafting,  293,  349;  effects  scion  on  stock, 

350;  437 

Graft  hybrids,  351 
Grand  period  of  growth,  337 
Grape  sugar,  27;  107 
Graphs;  of  transpiration,  203;  of  growth, 

330,  337 

Grappling  Plant,  395,  396 
Gravitation,  and  plants,  247;  as  stimulus, 

358,  359;  in  dissemination,  387 
Gray,  Asa,  6,  12,  71,  315 
Grayness  of  vegetation,  263 
Green  AlgEe,  446,  454 
Green  color  in  plants,  17 
Green-manuring,  124 
Green  Roses,  376 
Grew,  Nehemiah,  158,  220 
Growth,  145;  327;  operations,  327;  de- 


velopment, 327,  340;  enlargement,  327, 
328;  maturation,  327,  345;  measure-: 
ment,  328;  graphs  of,  330,  331,  337; 
and  temperature,  332,  333;  and  light, 
334,  335;  and  transpiration,  335;  and 
humidity,  336;  and  electricity,  336; 
and  poisons,  336;  and  ether,  336;  and 
barometric  pressure,  336;  of  leaves, 
roots,  stems,  338,  339,  340 ;  shortening, 
338;  minor  phenomena,  345;  move- 
ments, 346;  cell  division,  341;  water 
in,  343;  and  osmotic  pressure,  342; 
structural  phenomena,  352;  cycles, 
352;  egg-cell  to  embryo,  353;  germina:- 
tion,  358;  seedling,  359;  to  adult,  361:; 
embryonic  vs.  permanent  tissue,  362- 
secondary  growth,  362;  system,  363; 
and  age,  366;  seasonal  cycles,  367; 
disturbance,  369;  monstrous,  373;  dis- 
semination by,  381 

Guard  cells,  action,  207,  208 

Gum  arabic,   114 

Gums,  chemistry,  114 

Guttation,  210 

Gymnosperms,  460,  461 

Habenaria,  fertilization,  312,  313 

Hairs,  389 

Hales,  Stephen,  5,  223 

Halophytes,  464 

Heat,  from  respiration,  90;  protection 

against,  265 
Heliotropism,  227 

Hemispherical  shape,  explained,  50,  51 
Herbaria,  2 
Heredity,  408 
Hildebrand,  401 
Hooke,  Robert,  158 
Hooks  on  seeds,  and  fruits,  70,  395,  396 
Horsetails,  457 
Horticulture,  3 
Hot  springs,  plants  in,  265 
House  plants,  how  determined,  201 
Humidity  and  growth,  326 
Humus,  122 
Huxley,  12,  141,  412 
Hybridization,  nature,  437, 439;  practice, 

441 


472 


Index 


Hybrid  lilies,  440 
Hybrids,  characters,  440 
Hydrolysis,  217 
Hydrophytes,  464 
Hydrotropism,  239,  240 
Hygrometers  and  bygroscopes,  189 
Hygroscopic  movements,  385 
Hygroscopicity,   explained,   188;  move- 
ments of,  189 
Hypocotyl,  354 
Hypothesis,  use  of,  32 
Hysterophytes,  465 

Imbibition,  explained,  178;  187,  215 

Immunity,  273 

Improvement,  illustration,  428;  hypo- 
thetical case,  434 

Inbreeding,  438 

Incidental  phenomena,  347 

Indian  Pipe,  453 

Indigo,  119 

Initial  cell,  353 

Insanity,  357 

Insectivorous  plants,  69,  194,  245, 
464 

Insect  pollination,  309,  461 

Insect  traps,  69 

Insects,  and  flowers,  317,  323;  resem- 
bling seeds,  399 

Intelligence  in  Man,  14;  in  Nature,  14 

Intercellular  system,  190 

Internodes,  explained,  61 

Invertase,  129 

Iodine  test  for  starch,  23,  109 

Iris,  flower,  316 

Irish  Moss,  447 

Irritability,  145,  148;  nature  of,  224; 
227;  elements  in,  227;  nature  of  re- 
sponses, 229;  stimulus,  229;  localized 
perception,  234;  motor  mechanism, 
228;  vs.  heredity,  237;  in  plant  im- 
provement, 431 

Isolation  of  biotypes,  432 

Ivory  Palm,  112,  156;  cellulose  of, 
113 

Japanese  Air-plant,  458 
Juvenescence,   301 


Kelps,  447 
Kerner,  314,  401 
Knuth,  314 

Laboratory  methods,  value  of,  138 

Lamarck,  biography,  412 

Lathyrus  Aphaca,  68 

Leaf  fall,  40 

Leaves,  anatomy,  19,  21,  Plate  I;  weight, 
25;  why  exist,  50;  characteristics,  51; 
parts,  52 ;  typical,  53 ;  variety  of  shapes, 
53,  54;  typical  shapes,  55,  66;  con- 
ventionalizations, 57;  terminology  of 
shapes,  58;  emarginations,  59;  com- 
pounding, 59;  lobing,  60;  gas  move- 
ments in,  84;  sizes,  65,  258;  compound- 
ing, 259;  thickness,  259 

Leaf  mosaics,  233;  235 

Lecithins,  117 

Lenticels,  191 

Leucoplastids,  160 

Lichens,  451 

Life,  nature,  viii,  13,  14;  characteristics 
of,  95;  in  relation  to  carbon,  96;  energy 
changes,  104,  134;  two  elements  in, 
144;  origin  of,  96,  149;  cycles,  163; 
rejuvenation,  163 

Life  in  relation  to  carbon,  96;  all  earth 
can  support,  104 

Life-plants,  392 

Light,  nature,  32;  use  in  photosynthesis, 
34,  48,  50;  on  autumn  color,  148;  ad- 
justment to,  224,  225,  235;  protec- 
tion against,  256,  261;  and  growth, 
334 

Light  screens,  39,  43,  262,  455 

Lignin,  113,  157 

Linaria  Cymbalaria,  237 

Linden,  fruits,  389 

Linnsea,  326 

Linnaeus,  quoted,  1 1 

Lipase,  128 

Liquors,  origin,  98 

Liverworts,  393,  454 

Living  Oats,  385 

Living  plant,  an  energy  station,  198 

Living  protoplasm,  chemistry,  106,  130 

Locomotion  of  plants,  304,  379 


Index 


473 


Long  Moss,  456 
Lotus,  394 
Lubbock,  315 

Machinery  of  photosynthesis,  36,  37;  of 
respiration,  89;  of  absorption,  169;  356 

Macrocystis,   445 

Madder,  119 

Malaria,  201 

Male,  meaning  of,  291 

Malpighi,  158 

Manuals,  2 

Manufacture  of  sugar,  35 

Maple,  sugar,  108;  fruits,  388 

Margins  of  leaves,  59 

Maturation,  327,  345 

Mechanical  causation,  vi,  65;  responses, 
245 

Mechanism,  viii,  13 

Medullary  rays,  222,  365 

Membranes,  nature  of,  173,  176,  dia- 
grams of,  177 

Mendel,  295;  his  Law,  295;  diagram,  297; 
work,  417;  in  evolution,  422,  439 

Meristem,  340,  362 

Mesophytes,  464 

Metabolism,  105,  145 

Methods  of  study,  1 

Micellae,  explained,  146,  176;  diagrams 
of,  177 

Microbes,  451 

Micropyle,  281 

Mildews,  273,  451 

Mimicry,  275,  399 

Mind,  in  research,  10 

Minerals,  in  photosynthesis,  48;  in  plants, 
134,  136;  absorption  by  plants,  189; 
as  excretions,  212 

Mistletoe,  397,  453 

Mitochondria,  159 

Mobility,  145 

Mohl,  H.  von,  156,  158,  159 

Molds,  273,  451 

Molecular  vs.  molar  forces,  194 

Molecules,  175 

Monocotyledons,  461 

Monstrosities,  357,  373,  375,  436 

Morphine,  125 


Morphology,  defined,  2,  67;  vs.  ecology, 

73;  diagram,  74;  404 
Mosses,  454 
Moss-plants,  454 
Moth,  pollinator,  314 
Mucilage;  157;  on  seeds,  396,  400 
Mucilaginous  modification,  113 
Muehlenbeckia,  71 
Miiller,  314 
Multiplication,  301 
Mummy  seeds,  355 
Muscarine,  125 
Mushrooms,  451 
Mutants,  419,  432 
Mutation,  explained,  419 
Mycelia,  381,  452 

Mycorhiza;  absorption  by,  193;  451 
Myrmecophila,  464 

Nastic  movements,  252 

Natural  History  of  Plants,  3 

Natural  Selection,  257;  explained,  406 

Navel  oranges,  375,  436 

Nectar,  use,  309;  322;  glands,  310 

Nectaries,  extra-floral,  325 

Nectarines,  436 

Nelumbium,  396 

Nepenthes,69 

New  organs,  origin,  73 

Nicotine,  125 

Nitrification  of  soils,  121,  123 

Nitrifying  Bacteria,  122,  123 

Nitrogen;  assimilates,  106,  120;  in  at- 
mosphere, 120;  in  plants,  120;  ex- 
cretion, 125;  activity,  126 

Nodes,  explained,  61,  381 

Nodules  of  Leguminossp,  123,  124 

Non-adaptive  features,  13 

Non-green  plants,  17 

Nucleo-proteins,  127 

Nucleolus,  150 

Nucleus,  structure,  142;  150 

Oak  tree,  52 

Observation,  use  of,  18,  31 

Odor,  in  flowers,  321 ;  vs.  color,  322 

(Edema,  186 

(Enothera  Lamarckiana,  418,  419,  423 


474 


Index 


Oil  gland,  118 

Oils,  274 

Optical  sections,  19 

Orchid  seeds,  389 

Origin  of  protoplasm,  148 

Osmoscope,  171 

Osmosis,  experiment,  170;  permeable 
and  semi-permeable  membranes,  173; 
pressures,  182;  explained,  184;  dia- 
gram of,  184;  phenomena,  185,  186 

Osmotic  absorption,  173,  268;  turgcs- 
cence,  52;  pressures,  amount,  182;  ex- 
planations, 183;  phenomena,  185 

Ovary,  281 

Overproduction,  407 

Ovules,  281,  283;  changed  to  leaves,  375 

Oxygen,  in  photosynthesis,  30,  31;  in 
respiration,  82 

Pangenesis,  415,  417 

Parasites,  17;  and  growth,  370;  450,  465 

Parthenogenesis,  299 

Partridge  Berry,  317 

Pasteur,  148 

Pathology,  defined,  3 

Pavements  burst,  78,  79 

Pectins,  115 

Peptones,  127 

Perception,  229 

Perennials,  explained,  366 

Personification  of  Nature,  326 

Petioles,  52,  70;  function,  258 

Pfeffer's  cell,  182 

Phamotypes,  423 

Pharmacology,  3 

Philosophy  of  nature,  403 

Phosphoproteins,  127 

Phosphorus  in  plants,  126,  128 

Photonasty,  252 

Photosynthate,  defined,  24;  quantities, 
25 

Photosynthesis;  defined,  24;  product,  24; 
27;  materials  used,  28;  carbon  dioxide, 
29;  water,  29;  oxygen,  30;  equation, 
31;  quantities,  26,  26,  31;  and  chlo- 
rophyll, 32;  and  light,  32,  34;  as  manu- 
facture, 35;  visualization,  36;  machin- 
ery, 36,  37,  Plate  I;  adaptations 


thereto,  47,  50;  the  four  essentials,  48; 
effects  on  plant  form,  48 

Photosynthetic,  equation,  31;  tree,  61; 
sugar,  106;  its  fate,  132 

Phototropism,  explained,  226,  226;  ex- 
periment, 226;  negative  and  trans- 
verse, 227,  232;  of  leaves,  225;  of  stems, 
225;  of  roots,  226;  of  flowers,  234;  of 
fruits,  235 

Phyllotaxy,  described,  62;  systems,  62, 
63;  explanation,  64,  360 

Physiological  method,  203 

Physiology,  defined,  2 

Pineapple,  fasciated,  374 

Pistils,  open,  375 

Pitcher  leaves,  69,  375 

Pitcher  Plants,  68,  69,  194 

Pith,  222 

Plant  breeding,  defined,  3;  273;  404,  426; 
products,  426;  by  selection,  429;  pres- 
ervation of  sports,  435;  crossing  and 
hybridization,  437;  cultivation,  442; 
theory,  443 

Plant  fats,  116 

Plant  Industry,  defined,  3 

Plant  improvement,  methods,  428 

Plant  oils,  116;  in  seeds,  116 

Plants,  kinds  of,  1 

Plasomen,  146 

Plastids,  110,  150,  160 

Pleasures  of  Botany,  5,  6,  7,  46;  of 
science,  204,  217 

Pleurococcus,  446 

Plumes,  389 

Plumule,  354 

Poison  Ivy,  117,  275 

Poisons,  106,  125;  and  growth,  336 

Pollen,  damaged  by  water,  272;  281; 
male  cell,  282,  284;  grains,  described, 
306;  protection  of,  324;  projected, 
323;  tube,  guided,  241;  growth,  282, 
291 

Pollination,  282 

Poppy  pods,  387 

Potash  in  plants,  135,  136 

Potential  vs.  kinetic  energy,  93 

Potted  plants,  care  of,  87 

Pressure  by  roots,  78 


Index 


475 


Projection  of  seeds,  383,  384,  385 

Prolamins,  127 

Proliferation,  374,  376 

Propulsion,  of  water  up  stems,  215 

Protease,  128 

Protection,  256;  adaptations,  257; 
against  winds,  257;  against  light,  261; 
against  weight  of  snow  and  ice,  259; 
against  heat,  265;  against  dryness, 
267;  against  too  much  water,  271; 
against  parasitic  plants,  273;  against 
animals,  274 

Proteins,  106,  126;  chemistry,  126; 
grains,  127 

Proteoses,  127 

Prothallia,  458 

Protista,  448 

Protoplasm;  in  cells,  22;  a  real  sub- 
stance, 138;  appearance,  139,  140, 143; 
146;  streaming,  140,  141;  texture,  141; 
pictures  of,  142;  chemistry  of,  142, 
144,  147;  lability  of,  143;  par  excel- 
lence, 143;  regulatory  power  in,  144; 
physiological  properties  of,  145;  af- 
fected by  external  conditions,  147; 
origin  of,  148;  organization  of,  149; 
identity  in  animals  and  plants,  153, 
continuity  of,  157;  named,  159;  why 
separated  into  cells,  161 ;  rejuvenation 
of,  163;  vitality  suspended,  164;  pro- 
tection of,  257 

Protoplasmic  streaming,  77 

Pteridophytes,  456 

Ptomaines,  103 

Puff-balls,  451 

Pulp  of  fruits,  398 

Purity  of  germ  cells,  298 

Purposefulness,  vii,  11,  12 

Quartered  oak,  365 
Quinine,  125 

Radial  structure,  49 

Radium,  effects  on  plants,  251 

Rain,  protection,  324 

Rancification,  101 

Reason,  255 

Red  Alga;,  289,  447 


Red  Snow,  447 

Redness  of  spring  vegetation,  263 

Reduction;  in  size,  390;  division,  285, 
298,  422;  of  surface,  271 

Reflex  action,  255 

Regulatory  power  in  life,  144 

Regulators  of  metabolism,  106,  128 

Reindeer  Moss,  456 

Rejuvenescence,  163,  367 

Reproduction,  278;  asexual  described, 
279;  sexual  described,  281;  in  relation 
to  sex,  286;  structures,  286;  in  relation 
to  characters,  293;  Mendelian  basis, 
295;  balance  with  vegetation,  301 

Resins,  274 

Respiration;  balance,  31;  nature,  80; 
experiment,  81,  82;  release  of  carbon 
dioxide,  81;  absorption  of  oxygen,  82; 
vs.  photosynthesis,  83,  94,  95;  quan- 
tities, 83;  vitiates  air,  84;  balance  with 
photosynthesis,  84,  85;  vs.  breathing, 
85;  chemistry,  88;  equation,  88;  ob- 
ject, 88;  vs.  combustion,  89,  90;  ma- 
chinery, 89;  release  of  heat,  90;  loss 
of  weight,  91;  source  of  energy,  92;  re- 
lation to  fermentation,  100;  anaerobic, 
102;  intramolecular,  102;  relation  to 
decay,  102;  relation  to  disease,  102; 
and  cell  size,  162;  and  movements,  228 

Respiratory  equation,  88 

Resting  state,  357 

Resurrection  plant,  392 

Reversions,  375 

Rheotropism,  251 

Rockweeds,  289,  447 

Rogues,  438 

Roots;  characteristics,  66;  as  foliage,  71; 
drown,  87;  excretions,  125;  structure, 
165,  195;  hairs,  165,  166;  anatomy, 
167;  cap,  166;  growing  point,  166; 
absorption  by,  172;  adaptations,  196; 
morphological  modifications,  197;  ori- 
gin of,  196;  poisons,  212;  pressures  ex- 
erted, 78,  214;  prop,  258;  shortening, 
382. 

Rose  of  Jericho,  392 

Rots,  273,  451 

Rubber,  119 


476 


Index 


Rudimentary  structures,  405 
Runners,  381 
Rusts,  273,  451 

Sachs,  32,  215 

Salt  marshes,  271 

Salvia,  fertilization,  312 

Sand  Box,  384 

Sap-cavities,  150,  161;  growth,  342 

Saprophytes,  17,  450,  465 

Sargassum,  447 

Scientific  procedure,  31 

Scion,  or  cion,  349 

Scorpiurus,  399 

Sea-mosses,   447 

Secondary  sexual  characters,  288,  291 

Secretions,  106,  116,  220 

Sedentary  habit,  49,  378 

Seedling,  described,  359,  361 

Seed-plants,  described,  459 

Seeds,  364;  vitality,  355;  from  mummies, 
355;  protection  o.",  276;  animal  car- 
riage, 394;  dissemination,  378;  pro- 
jection, 383;  waftage,  387;  wings,  388; 
notation,  393;  vs.  fruit,  391 

Seed  sports,  437 

Selection  of  variations,  429 

Self-planting,  400 

Semi-permeable  membranes,   173 

Sense  organs,  234 

Sensitive  Plant,  237,  243,  244,  251 

Sensitivity,  145 

Sex,  278;  meaning  of,  286,  291;  origin 
of,  288;  prominence  of,  292 

Sexual  reproduction,  significance,  286; 
superior  to  asexual,  293 

Sexual  cells,  280,  288 

Shapes  of  leaves,  53 

Shoot  and  root  explained,  49 

Sieve  tubes,  described,  155,  219,  221 

Skeleton  of  plants,  106,  107, 152,  257 

Sleep  movements,  236,  238,  244,  266 

Sleeping  rooms,  plants  in,  87 

Slime,  or  jelly,  of  plants,  273 

Slime  molds,  141,  449 

Slowness  of  plant  actions,  76 

Smilax,  67,  71 

Smuts,  273,  451 


Snails,  in  cross  pollination,  324 

Society,  plant,  465 

Soils,  aeration,  85;  structure,  86,  122; 
drainage,  87 

Spanish  Moss,  455 

Special  Creation,  403 

Spectroscope,  described,  33 

Spermatozoids,  283,  290 

Spermatophytes,  described,  459 

Sphagnum,  455 

Spines,  68;  use,  275 

Spirogyra,  304,  446 

Spongiole,  169 

Spontaneity  of  variation,  432 

Spontaneous  generation,  148 

Spores,  colors,  263,  asexual,  280,  281 ;  in 
air,  391;  446,  453,  455,  457 

Sports,  preservation,  435 

Spots,  451 

Sprengel,  314 

Spring,  coloraton,  39;  vegetation  in,  368 

Spruce  tree,  59 

Squirting  Cucumber,  384,  386 

Stamens,  sensitive,  323 

Starch,  in  leaves,  23,  tests,  23,  chemistry, 
27,  108, 110;  iodine  test,  109;  as  reserve 
food,  109;  food  for  man,  109;  grains, 
structure,  110,  112;  digestion  of,  110; 
individuality  of,  111;  in  potato,  111 

Stems,  characteristics,  61;  with  func- 
tion of  foliage,  70,  71;  generalized, 
214;  two  typos,  364;  cellular  anatomy, 
220,  221;  construction,  222.  258 

Sterilization  methods,  103 

Stigma,  281;  sensitive,  323 

Stimulus,  229;  perception  of,  229;  dif- 
ferential responses,  230,  231 ;  action  of, 
242,  247,  248,  253,  255;  in  growth, 
355,  358 

Stipa  pinnata,  400 

Stipules,  53,  70 

Stolons,  381 

Stomata,  22;  number,  207,  size,  207; 
use  of,  269,  273 

Stomatal  chambers,  271 

Storage  battery,  93,  96 

Strains,  adjustments  to,  252 

Strand  plants,  465 


Index 


477 


Streaming  of  protoplasm,  140,  141 

Struggle  for  existence,  407 

Strychnine,  125 

Style,  281 

Suberin,  157 

Substances  made  by  plants,  105 

Substitution  foliage,  70 

Succulent  plants,  269 

Suckers,  381 

Sucrose,  108 

Sugar,  in  leaves,  24,  27;  various,  108 

Sulphur  in  plants,  126,  128 

Sulphur  showers,  309 

Summer,  vegetation  in,  368 

Sundew,  245 

Super-vitalism,  14,  96 

Survival  of  fittest,  408 

Suspensor,   353 

Symmetry;  in  form,  238;  250;  in  growth, 

369 
Systematic  Botany,  defined,  2 

Tannins,  118,  274 

Taxis,  252 

Teleology,  nature,  12 

Temperature,  and  growth,  332 

Tendrils,  67,  68,  243 

Thermonasty,   252 

Thermotropism,  251 

Thigmotropism,  242,  243 

Thought  in  nature,  147,  402 

Tissues,  157 

Toadstools,  451 

Tone,  253 

Torsions,  374 

Toxicodendrol,    117 

Traction,  of  water  up  stems,  216 

Trailers,  464 

Translocation  of  food,  217;  in  bark,  219 

Transmission  of  acquired  characters, 
413 

Transpiration,  described,  199;  Experi- 
ment, 199;  quantity,  200;  determines 
phenomena,  202;  variations  in  amount, 
203;  effect  of  heat,  204,  of  light,  206, 
208,  of  dryness,  206,  of  wind,  206; 
graph,  205;  meaning,  209;  and  growth, 
335 


Transpirograph,  203,  204 
Traumatropism,  251 
Tree  forms,  51,  58,  262 
Tree  of  ascent,  449 
Tropisms,  251 
Tulip  Tree,  stipules,  67 
Tumble-weeds,  392 
Tumors,  371 
Turf,  382 
Turgescence,  384 
Twin  flower,  326,  397 
Twisted  stems,  374 
Typical,  meaning  of,  9 

Undergrowth  plants,  456,  460 

Unicorn  Plant,  396 

Unit  characters,  421 

Unity  of  science,  7 

Uroglsena,  446 

Useless  vs.  useful  science,  4 

Utility  of  science,  4,  5 

Vallisneria  spiralis,  305 

Variations;    nature,    407;    selection    of, 

429;  experimental,  430;   innate,  431; 

hereditary,    432;    spontaneous,    432; 

fortuitous,   433 
Variegated  plants,  436 
Vascular  System,  458 
Vaucheria,  446 
Vegetable  balls,  376 
Vegetable  ivory,  112 
Vegetables,  improvement  of,  428 
Vegetative  multiplication,  279 
Veins,  52,  53,  221 
Venation,  53 
Venus  Flytrap,  245 
Verities;  nature  of,  9;  18,  24,  28,  35,  80, 

85,  92,  95,  103,  172,  176,  184 
Vertical  position,  263 
Vetch,  pods,  383 
Violet,  flowers,  318;  pods,  386 
Visualization     of     photosynthesis,     36; 

219,  251 

Vitalism,  vii,  viii,  13,  14,  96 
Vitality  suspended,   164 
Volatile  oils,   117 


478 


Index 


Waft  age  of  seeds,  387 

Walking  Fern,  381 

Warping  of  wood,  187;  diagram,  188 

Water,  in  photosynthesis,  30,  48;  storage, 
269;  protection  against,  256,  271 

Water  culture,  methods,  135,  136 

Water-lily  seed,  394 

Water-molds,  451 

Water-plants,  aeration  system,  195; 
448;  464 

Water-rolled  balls,  376 

Waxes,  118 

Wearing  out  of  varieties,  443 

Wcismann,  415,  423 

Wild  Geranium,  385 

Wind  pollination,  307 

Winds,  in  pollination,  305;  in  dissemina- 
tion, 387;  protection  against,  256;  257 


Wings  of  seeds  and  fruits,  70,  388 
Winter-killing,  202 
Winter,  vegetation  in,  367 
Witches  Brooms,  371 
Wooden  flowers,  371 
Work  of  plants,  76,  examples,  77;  reality, 
79 

Xanthophyll,  41;  in  autumn  leaves,  41 
X-rays,  on  plants,  251 
Xenia,  300 
Xerophytes,  465 

Yeast,  97,  451 


Zoospores,  280,  380 
Zymase,   129 


THE  AMERICAN  NATURE  SERIES 

In  the  hope  of  doing  something  towards  furnishing  a  series  where  the 
nature-lover  can  surely  find  a  readable  book  of  high  authority,  the  pub- 
lishers of  the  American  Science  Series  have  begun  the  publication  of  the 
American  Nature  Series.  It  is  the  intention  that  in  its  own  way,  the  new 
series  shall  stand  on  a  par  with  its  famous  predecessor. 

The  primary  object  of  the  new  series  is  to  answer  the  questions  which 
the  contemplation  of  Nature  is  constantly  arousing  in  the  mind  of  the 
unscientific  intelligent  person.  But  a  collateral  object  will  be  to  give  some 
intelligent  notion  of  the  "causes  of  things." 

While  the  co-operation  of  foreign  scholars  will  not  be  declined,  the 
books  will  be  under  the  guarantee  of  American  experts,  and  generally 
from  the  American  point  of  view ;  and  where  material  crowds  space, 
preference  will  be  given  to  American  facts  over  others  of  not  more  than 
equal  interest. 

The  series  will  be  in  six  divisions  : 

I.     NATURAL  HISTORY 

This  division  will  consist  of  two  sections. 

Section  A.  A  large  popular  Natural  History  in  several  volumes, 
with  the  topics  treated  in  due  proportion,  by  authors  of  unquestioned 
authority.  8vo.  TjxlO^  in. 

The  books  so  far  publisht  in  this  section  are: 

FISHES,  by  DAVID  STARR  JORDAN,  President  of  the  Leland  Stanford 
Junior  University.  $6.00  net;  carriage  extra. 

AMERICAN  INSECTS,  by  VERNON  L.  KELLOGG,  Professor  in  the  Leland 
Stanford  Junior  University.  $5.00  net;  carriage  extra. 

BIRDS  OF  THE  WORLD.  A  popular  account  by  FRANK  H.  KNOWLTON, 
M.S.,  Ph.D.,  Member  American  Ornithologists  Union,  President 
Biological  Society  of  Washington,  etc.,  etc.,  with  Chapter  on  Anat- 
omy of  Birds  by  FREDERIC  A.  LUCAS,  Chief  Curator  Brooklyn  Muse- 
um of  Arts  and  Sciences,  and  edited  by  ROBERT  RIDGWAY,  Curator  of 
Birds,  U.  S.  National  Museum.  $7.00  net;  carriage  extra. 

Section  B.  A  Shorter  Natural  History,  mainly  by  the  Authors  of 
Section  A,  preserving  its  popular  character,  its  proportional  treatment,  and 
its  authority  so  far  as  that  can  be  preserved  without  its  fullness.  Size  not 
yet  ietermined. 

II.     CLASSIFICATION  OF  NATURE 

Section  A.    Library  Series,  very  full  descriptions.    8vo.     7|xlOiin. 

Already  publisht; 
NORTH  AMERICAN  TREES,  by  N.  L.  BRITTON,  Director  of  the  New 

York  Botanical  Garden.      $7.00  net;  carriage  extra. 
FERNS,   by    CAMPBELL   E.    WATERS,  of  Johns   Hopkins  University.      8vo, 

pp.  xi+362.      Price  $3.00  net;  by  mail,  $3.30. 

Section  B.  Pocket  Series,  Identification  Books—  "  How  to 
Know,"  brief  and  in  portable  shape. 


AMERICAN     NATURE     SERIES     (Continued) 

III.    FUNCTIONS  OF  NATURE 

These  books  will  treat  of  the  relation  of  facts  to  causes  and  effects — 
of  heredity  and  the  relations  of  organism  to  environment.      6§ x8&  in. 
THE  BIRD:  ITS  FORM  AND  FUNCTION,  by  C.  W.  BEEBE,    Curator 

of  Birds  in  the  N.  Y.  Zoological  Park.  496  pp.  $3. 50  net ;  by  mail,  $3. 80. 
THE  LIVING  PLANT,  by  WILLIAM  F.  GANONG,  Professor  in  Smith 

College.      $3.50  net;  by  mail,  $3.80. 

IV.      WORKING   WITH    NATURE 

How    to    propagate,    develop,    care    for    and    depict    the  plants  and 
animals.      The  volumes  in  this  group  cover  such  a  range  of  subjects  that  it 
is  impracticable  to  make  them  of  uniform  size. 
NATURE  AND  HEALTH,  by  EDWARD  CURTIS,  Professor  Emeritus  in  the 

College  of  Physicians  and  Surgeons.  12mo,  $1.25  net;  by  mail,  $1.37. 
THE  LIFE  OF  A  FOSSIL  HUNTER,  by  CHARLES  H.  STERNBERG. 

Large  12mo,  $1.60  net;  by  mail,  $1.72. 
THE  FRESHWATER  AQUARIUM    AND    ITS    INHABITANTS.      A 

Guide  for  the  Amateur   Aquarist,  by  OTTO  EGGELING  and  FREDERICK 

EHRENBERG.      Large  12mo,   $2. 00  net;  by  mail,  $2.19. 
THE  SHELLFISH  INDUSTRIES,    by  JAMES   L.  KELLOGG,  Professor  in 

Williams  College.      Large  12mo,  $1.75  net;  by  mail,  $1.93. 
THE  CARE  OF  TREES  IN  LAWN,  STREET  AND  PARK,  by  B.  E. 

FERNOW,   Professor  of  Forestry  in  the  University  of  Toronto.      Large 

12mo,  $2.00  net;  by  mail,  $2.17. 
HARDY   PLANTS  FOR  COTTAGE  GARDENS,  by  HELEN  R.  ALBEE. 

Large  12mo,  $1.60  net;  by  mail,  $1.73. 
INSECTS   AND  DISEASE,     by   RENNIE  W.   DOANE,    Assistant  Professor 

in  the  Leland  Stanford  Junior  University.   $1.50  net;  by  mail,  $1.62. 

V.      DIVERSIONS    FROM    NATURE 

This  division  will  include  a  wide  range  of  writings  not  rigidly  system- 
atic or  formal,  but  written  only  by  authorities  of  standing.  Large  12mo. 
5i\8i  in. 

INSECT  STORIES,   by  VERNON  L.  KELLOGG.     $1.50  net;  by  mail,  $1.62. 
FISH  STORIES,  by  CHARLES  F.  HOLDER  and  DAVID  STARR  JORDAN.    $1.75 

net;  by  mail,  $1.87. 

VI.     THE    PHILOSOPHY    OF    NATURE 

A   Series  of  volumes  by    President   JORDAN,   of  Stanford  University, 
and   Professors  BROOKS  of  Johns  Hopkins,  LULL  of  Yale,  THOMSON  of  Aber- 
deen, PRZIBRAM  of  Austria,  ZUR  STRASSEN  of  Germany,  and  others.     Edited 
by  Professor  KELLOGG  of  Leland  Stanford.     12mo.      5^x7^  in. 
THE  STABILITY  OF  TRUTH,  by  DAVID  STARR  JORDAN.     $1.25  net; 

by  mail,  $1.33. 
PLANT  LIFE  AND  EVOLUTION,  by  D.  H.   CAMPBELL.      $1.75  net; 

by  mail,  $1.92. 

Arranged  for: 
THE  CONTROL  OF   LIFE,  by  J.  ARTHUR  THOMSON. 

HENRY     HOLT      AND      COMPANY,      NEW  YORK 

MAR.    '13. 


UC  SOUTHERN  REGIONAL  LIBRARY  FACILITY 

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